Diamond

ABSTRACT

A method of producing a CVD single crystal diamond layer on a substrate includes adding into a DVD synthesis atmosphere a gaseous source comprising silicon. The method can be used to mark the diamond material, for instance to provide means by which its synthetic nature can more easily be determined. It can also be exploited to generate single crystal diamond material of high colour.

This is continuation-in-part application of U.S. application Ser. No.______, filed Jun. 12, 2006, which is a National Stage ofPCT/IB04/04069, filed Dec. 10, 2004.

BACKGROUND OF THE INVENTION

This invention relates to a method of marking or fingerprinting diamondmaterial, in particular single crystal synthetic diamond materialproduced by chemical vapour deposition (hereinafter referred to as CVD),thereby providing a mark of origin or fingerprint in the diamondmaterial, or a means by which its synthetic nature can more easily bedetermined. This method can also be exploited to generate single crystaldiamond material of high colour.

Methods of depositing material such as diamond on a substrate by CVD arenow well established and have been described extensively in patent andother literature. Where diamond is being deposited on a substrate, themethod generally involves providing a gas mixture which, ondissociation, can provide hydrogen or a halogen (e.g. F, Cl) in atomicform and C or carbon-containing radicals and other reactive species,e.g. CH_(x), CF_(x) wherein x can be 1 to 4. In addition, oxygencontaining sources may be present, as may sources for nitrogen, and forboron. Nitrogen can be introduced in the synthesis plasma in many forms;typically these are N₂, NH₃, air and N₂H₄. In many processes inert gasessuch as helium, neon or argon are also present. Thus, a typical sourcegas mixture will contain hydrocarbons C_(x)H_(y) wherein x and y caneach be 1 to 10 or halocarbons C_(x)H_(y)Hal_(z) wherein x and z caneach be 1 to 10 and y can be 0 to 10 and optionally one or more of thefollowing: CO_(x), wherein x can be 0.5 to 2, O₂, H₂, N₂, NH₃, B₂H₆ andan inert gas. Each gas may be present in its natural isotopic ratio, orthe relative isotopic ratios may be artificially controlled; for examplehydrogen may be present as deuterium or tritium, and carbon may bepresent as ¹²C or ¹³C. Dissociation of the source gas mixture is broughtabout by an energy source such as microwaves, RF (radio frequency)energy, a flame, a hot filament or jet based technique and the reactivegas species so produced are allowed to deposit onto a substrate and formdiamond.

CVD diamond may be produced on a variety of substrates. Depending on thenature of the substrate and details of the process chemistry,polycrystalline or single crystal CVD diamond may be produced.

The development in the level of sophistication of methods of producingCVD single crystal diamond has meant that this material is becomingincreasingly more suitable for use in industrial applications or inornamental applications such as synthetic gemstones for jewellery.However, in many applications there is a need to provide a method ofdetermining the source of synthetic diamond used in these applicationsin order to verify its origins or synthetic nature.

SUMMARY OF THE INVENTION

According to one aspect of the invention, there is provided a method ofincorporating a mark of origin, such as a brand mark, or fingerprint ina CVD single crystal diamond material, which includes the steps ofproviding a diamond substrate, providing a source gas, dissociating thesource gas thereby allowing homoepitaxial diamond growth, andintroducing in a controlled manner a dopant into the source gas in orderto produce the mark of origin or fingerprint in the synthetic diamondmaterial, which dopant is selected such that the mark of origin orfingerprint is not readily detectable or does not affect the perceivedquality of the diamond material under normal viewing conditions, butwhich mark of origin or fingerprint is detectable or rendered detectableunder specialised conditions, such as when exposed to light or radiationof a specified wavelength, for example.

Detection of the mark of origin or fingerprint may be visual detectionor detection using specific optical instrumentation, for example.

The mark of origin or fingerprint is preferably provided in the form ofone or more layers or regions grown into the diamond material duringsynthesis.

Such a mark of origin or fingerprint in a CVD single crystal diamondmaterial is most appropriate in CVD diamond which is of high commercialor gem quality. Synthesis of such high quality CVD diamond material isbest performed using a diamond substrate having a surface on whichgrowth takes place which is substantially free of crystal defects, andthis forms a preferred version of the method of the invention.

The step of introducing a dopant into the source gas in a controlledmanner also leads to the generation of diamond of high colour. Forexample, the method of the present invention is able to provide a singlecrystal CVD diamond layer having high colour where the synthesisatmosphere comprises a gas having a first impurity atom type that wouldprevent high colour diamond from being produced. For example, thepresence of nitrogen gas in the synthesis atmosphere typically causesthe synthesised diamond to have a yellow/brown colour, whereas thepresence of boron gas in the synthesis atmosphere would typically causethe synthesised diamond to have a blue colour.

The second gas is deliberately added to the CVD diamond synthesisatmosphere. Preferably, the second gas is added in a controlled manner.The presence of the second gas may be controlled such that theconcentration of the second gas is stable to better than 20%, preferablybetter than 10%, preferably better than 3%.

Without being bound by any particular theory, it is believed that thesecond impurity atom type suppresses the roughening effect that thefirst impurity atom type otherwise has on the growth surface. By keepingthe growth surface smooth the uptake of a wide range of defects issuppressed which otherwise degrade the colour. Addition of a gaseoussource comprising an impurity atom type (such as nitrogen) to a singlecrystal CVD diamond synthesis process, can change the reactionsoccurring on the diamond growth surface in such a way that the roughnessof the surface is increased, giving the surface a greater propensity forincorporation of defects. This is particularly the case when an impuritycatalyses the nucleation of new layers of diamond in different regionsof a {100} surface, leading to the formation of macrosteps consisting ofterraces with inclined risers that offer different kinds of sites fordefect incorporation, such as is described in Martineau et al., Gems &Gemology, 40(1), 2 (2004).

There are many kinds of defects that may potentially be involved. Forexample, single substitutional impurity defects may be incorporated.These involve the substitution of a carbon atom by an impurity atom.Hydrogen is always present in the CVD growth environment and may becomeincorporated either on its own or in combination with one or moreimpurity atoms. Vacancies (unoccupied sites in the diamond lattice whichwould normally be occupied by a carbon atom) may become incorporated incombination with one or more neighbouring impurity atoms (e.g.nitrogen-vacancy defects), or one or more hydrogen atoms (e.g.vacancy-hydrogen complexes). Some defect complexes involve impurityatoms, hydrogen atoms and vacancies (e.g. nitrogen-vacancy-hydrogencomplexes). Clusters of vacancies may be formed with or without bondedhydrogen and in some cases may be associated with impurity atoms.

The wide ranging set of defects incorporated once the surface isroughened is generally found to have an undesirable effect on theoptical and electronic properties of the material. For example, the setof defects may contain some which give the material undesirable opticalproperties because of the way they absorb light in, for example, thevisible region of the spectrum. They will degrade electronic propertiesbecause they reduce the mobility and lifetime of carriers.

One general mechanism that is believed to underlie the current inventionis that the deleterious effect of one gaseous source comprising a firstimpurity atom type can be suppressed by the addition of a second gaseoussource comprising a second impurity atom type which suppresses theroughening effect that the first impurity atom type would otherwise haveon the growth surface. By suppressing the roughening of the surface theaddition of the second impurity also suppresses the incorporation of thewide range of defects outlined above that degrade the properties of thematerial grown.

In the presence of both impurity atom types, with growth taking place ona smooth surface, the two impurity atom types will generally beincorporated but with a lower efficiency than would be observed forgrowth on a rough surface. It is significant, however, that many of thedefects discussed above (e.g. vacancy clusters and hydrogen-relateddefects) are not observed at all when growth has taken place on a smoothsurface as a result of the addition of the second impurity atom type.The outcome is that the two impurity atom types may be incorporated intothe diamond material at moderate concentrations that are measurable butwithout the wide range of defects that have the strongest adverseeffects on the properties of the diamond layer produced, such as itsoptical transmission.

It is also believed that the method of the present invention mayadditionally be based on a second general principle in which the twoimpurity atom types are incorporated in such a way that they mutuallycompensate each other. As such, the two impurity atom types are chosenso that, within particular concentration ranges in the diamond layer,they do not have a substantial adverse effect on the material propertiesthat are desired. According to prior teaching in the art, there would bean assumption that this would exclude any benefit from compensationusing nitrogen, which is normally associated with a range of defectsthat degrades the colour and other properties. However, in light of thefirst general principle outlined above that defects can be decreased onaddition of a second impurity atom type, additional advantage can betaken of the mutual compensation effect between the two impurity atomtypes. This will generally be partly because one impurity atom typecompensates for the effect that the other would have in its absence andvice-versa. Compensation can be illustrated using the example ofnitrogen and boron. By themselves substitutional nitrogen and boron givediamond yellow/brown and blue colour, respectively. However, theinventors of the present application have found that when presenttogether in approximately the same concentrations, colourless materialcan result because the substitutional nitrogen defects donate electronsto the substitutional boron defects and the resultant ionised defects donot give rise to significant optical absorption.

For a given set of growth conditions (such as, substrate temperature,pressure and plasma temperature) the inventors have found that there isa threshold nitrogen concentration that can be tolerated by the CVDdiamond synthesis process before the surface roughens and the growndiamond becomes brown. However, the threshold nitrogen concentrationtends to be so low that considerable time and expense is involved toachieve a nitrogen concentration below the threshold in order to avoidthe incorporation of defects affecting the material's optical and otherproperties.

The inventors of the present invention have found that the addition of asecond impurity atom type (such as boron or silicon) to the growth gasescan significantly increase the threshold nitrogen concentration tolevels that might be present in growth environments when relativelylittle attention is given to nitrogen elimination. This allows diamondto be grown in the presence of relatively high concentrations ofnitrogen without the degradation of the optical and other propertiesthat would otherwise result because of the incorporation of defects suchas vacancy clusters and hydrogen-related defects. In addition it hasbeen surprisingly found that this is possible even though there may besignificant incorporation into the grown diamond of both nitrogen andthe second impurity atom type.

One aspect of the present invention thus relates to the use of a secondimpurity atom type to counter the detrimental effect on colour of afirst impurity atom type present in the CVD synthesis atmosphere. Inthis way, the present invention enables a CVD diamond to be producedwhich has high colour even though the CVD synthesis atmosphere comprisesan amount of a first impurity atom type which would otherwise produce adiamond not having high colour. This has the advantage of removing theneed to take special steps to eliminate impurity atom types known toadversely affect the colour of diamond from the synthesis atmospheremerely by adding a particular type and amount of a second impurity atom.Consequently, the synthesis of CVD diamond can be simplified and is moreefficient in both time and cost.

The CVD diamond layer produced by the method of the present invention issingle crystal diamond.

In one embodiment of the method of the invention, the dopant isnitrogen, which produces a mark of origin or fingerprint, preferably inthe form of a layer, that shows 575 nm and/or 637 nm luminescence peaks,with their associated vibronic systems, under suitable shorterwavelength excitation. The nitrogen doped layer may also show aphotoluminescence line at 533 nm.

In an alternative embodiment of the invention, the dopant is acombination of nitrogen and boron, where the boron is preferably presentin a higher concentration than the nitrogen, which produces a mark oforigin or fingerprint, preferably in the form of a layer, that generatescharacteristic phosphorescence, peaking typically in the range of 400 nmto 500 nm, under suitable shorter wavelength excitation. The addition ofa gaseous source comprising boron also counters the detrimental effecton colour of diamond that nitrogen would otherwise have.

In a particularly preferred embodiment of the invention, a combinationof layers that generate 575/637 nm luminescence and 400 nm to 500 nmphosphorescence under suitable shorter wavelength excitation is growninto the diamond material during synthesis.

A further alternative embodiment of the invention is the marking of alayer or region with the centre which emits 737 nm radiation underoptical excitation. The exact identity of this optical centre isuncertain, although it is believed to involve silicon; it will hereafterbe referred to as the 737 nm silicon related centre. Whereas theluminescence at 575/637 nm and the phosphorescence in the range 400 nmto 500 nm can easily be detected by eye under suitable viewingconditions, the detection of the luminescence from the 737 nm siliconrelated line is generally more easily detected using specific opticalinstrumentation with an integrated detector, and typically giving anoutput in the form of an analogue display. Additionally, by addingsilicon these impurity atoms counter the detrimental effect on colour ofdiamond of nitrogen impurity atoms.

In accordance with a further aspect of the invention there is provided aCVD single crystal diamond material bearing a mark of origin orfingerprint in the bulk thereof, which mark of origin or fingerprint isgrown into the diamond material during the synthesis thereof, preferablyin accordance with the method described above.

The single crystal diamond material may be prepared for a range ofindustrial applications. In particular, applications include those inwhich the synthetic diamond material is a visible element to the user,or where the synthetic diamond element is re-useable or needs periodicreprocessing, as is for example the case with diamond cutting bladessuch as scalpels.

Alternatively, the single crystal diamond material may be prepared orsuitable for preparation as a synthetic gemstone for jewelleryapplications.

The invention also extends to an apparatus for detecting the mark oforigin or fingerprint in a CVD single crystal diamond material, objector synthetic gemstone, the apparatus comprising:

-   -   a source of light or radiation of a particular wavelength for        causing excitation of the mark of origin or fingerprint,        resulting in luminescence and/or phosphorescence thereof; and    -   a detection means for detecting the mark of origin or        fingerprint, for example a viewer for viewing the luminescence        and/or phosphorescence, or an instrument providing a measure of        the intensity of the specific luminescence and/or        phosphorescence, in forms such as an analogue or digital        electrical signal, or display readout, for example.

The apparatus preferably comprises a range of optical filters forviewing the wavelengths emitted by the mark of origin or fingerprint,and means for excluding background white light or wavelengths used toexcite the fingerprint or mark of origin, or any other backgroundwavelengths present which may be detrimental to observing thewavelengths emitted by the mark. The characteristic luminescence and/orphosphorescence may be viewed in the form of a special image detectabledirectly in the diamond material, or it may be viewed by usinginstrumentation such as a charge coupled device (ccd) or imaging devicesuch as a digital camera. Alternatively, the luminescence and/orphosphorescence may be characterised by a spectroscopic device such asone or more specific band pass filters and/or frequency specificsensors, or a compact spectrometer. These techniques can be combined,for example using suitable filters in combination with a ccd camera toform frequency specific images.

The apparatus may also include magnification means for magnifying theimage of the synthetic diamond material.

DESCRIPTION OF PREFERRED EMBODIMENTS

The invention provides a method of marking synthetic diamond material,in particular such material prepared for use in industrial applicationssuch as cutting tools or as a CVD synthetic diamond gemstone. However,the method of the invention extends beyond applications involvingmarking of diamond and can be applied to any type of CVD single crystaldiamond.

The method of marking enables the determination of origin, said mark oforigin or fingerprint comprising one or preferably more layers growninto the diamond during synthesis, which do not substantially affect theperceived optical or gem qualities of the diamond under normal viewing,or significantly affect any other application specific propertiesrelating to the intended application, but which can be viewed underspecial viewing conditions. For convenience, the layer or layerscomprising this mark of origin or fingerprint may be referred to astagging layers.

Throughout this specification, the terms ‘fingerprint’ or ‘mark oforigin’ are deemed to include features which provide one or more of thefollowing benefits:

-   -   a) basic identification of the synthetic nature of the material;    -   b) identification of the manufacturer, or the method of        manufacture;    -   c) a brand mark or other characteristic mark;    -   d) a batch mark or date stamp;    -   e) a means by which post processing or modification of the        diamond or article formed from the diamond can be detected.

The fingerprint or mark of origin must also be relatively simple toapply to or incorporate in the material, and similarly be observed ordetected in a relatively unskilled operation using simple, low cost,compact and relatively portable equipment. By low cost, it is meant thatthe equipment is preferably <$10,000, and more preferably <$5,000, andeven more preferably <$2,000, and even more preferably <$1,000. On thisbasis, means of marking solely based on varying the isotopic ratios ofelements in the diamond away from natural abundance are specificallyexcluded. Isotope variation to mark material is known in a large rangeof materials, particularly in laboratory methods. In diamond suchmethods include two possible variants.

-   -   1) Varying the isotopic abundance of the carbon, i.e. using a        carbon source which is isotopically enriched in ¹²C or ¹³C. The        problem with this is twofold:        -   a. isotopically enriched ¹²C or ¹³C source gases are very            expensive, adding substantially to the cost of manufacture,            and        -   b. detection of isotopic variation requires complex and            expensive equipment and a high level of skill to operate and            interpret. Furthermore most techniques predominantly look at            surfaces and local features, rather than the bulk and            pattern of features throughout the bulk. Specific techniques            include secondary ion mass spectroscopy (which also damages            the sample), Raman analysis possibly combined with confocal            techniques, and high resolution X-ray diffraction, etc.    -   2) Varying the isotopic ratio of another element in the diamond        such as nitrogen. Such a technique is predominantly relevant to        HPHT synthetic diamond where the concentration of nitrogen and        other elements can be relatively high, for example nitrogen can        be in the range of 100-800 ppm. The problems with this are        similar to those with carbon isotope variation, in that:        -   a. isotopically enriched gases are expensive, adding to the            cost of manufacture        -   b. detection is complex, expensive and skilled. Detection is            further complicated in typical CVD diamond by the low            concentration of impurity elements in the diamond, often            below 1 ppm in the solid, so that some techniques which may            be applicable to HPHT diamond are not suitable for CVD            diamond.

Consequently, whilst in some instances there may be reasons to combinean isotopic variation with the method of this invention, for example toprovide additional features which are less easily detected, thisinvention excludes the required use of isotopic variation. Within thisspecification, isotopic dopants refers to dopants where the isotopicabundance is deliberately different from the natural isotopic abundance,so as to confer a detectable variation in isotopic abundance in thematerial. Chemical dopants refers to dopants which provide differentchemical elements, so as to confer a detectable variation in theelements within the material, at least in the form of defect centrescharacteristic of these other elements.

Furthermore, detection of the fingerprint or mark of origin underspecialized viewing conditions refers to detection of light of acharacteristic wavelength or colour emitted by the mark under thespecialized conditions and detected directly by the eye of the observer,or indirectly via optical detection means that then provide some meansof human detectable output, generally a visible analogue output althoughthis may be converted to an indication of whether the signal is above orbelow a threshold by the detection instrument. Generally the preferredmethod of detection is directly by the eye of the observer, since thisprovides the opportunity for spatial information, including binocular ordepth information, and provides a particularly inexpensive solution. Inthe case of one specific example given later, the 737 nm silicon relatedline, the eye is generally not sufficiently sensitive to detect thetypical levels of emission achieved, and a simple method of wavelengthselection and then optical energy detection may be preferred, and canstill be provided in simple, low cost, compact and relatively portableequipment.

By way of example, diamond scalpel blades are often re-useable,periodically returning to the manufacturer for preparation of a newedge. Used in this application, the mark of origin or fingerprint canfill one or more of the following functions, although its purpose maynot be limited to these examples:

-   -   a) Enable the specific manufacturer of the synthetic diamond        scalpel to be identified, either by the manufacturer or by the        marketplace. This can be used by the manufacturer to ensure that        only his own blades are accepted for reprocessing, and improve        the ability to track such blades within the reprocessing or in        the marketplace at large.    -   b) Provide a means by which to generate a distinctive mark, such        as a trademark, without degrading the material in its final        application. Ordinarily visible identification marks on the        synthetic diamond scalpel blade may not be acceptable for some        applications because of requirements of hygiene, uniform        transparency, or simply market expectation or acceptance.    -   c) Enhance the identification of the synthetic nature of the        diamond material. Synthetic diamond can offer greater        reproducibility and control in many industrial applications,        thus offering a better product.    -   d) Provide a means by which modification of the synthetic        diamond material may be identified, such modification including        changes to physical shape and annealing treatments such as those        which modify colour.

By way of further example, in the application of CVD diamond insynthetic gemstones, the mark of origin or fingerprint can fill one ormore of the following functions, although its purpose may not be limitedto these examples:

-   -   a) Enable the specific manufacturer of the CVD synthetic        gemstone to be identified, either by the manufacturer or by the        marketplace.    -   b) Provide a means by which to generate a distinctive mark such        as a trademark.    -   c) Enhance the identification of the synthetic nature of the        diamond material.    -   d) Provide a means by which modification of the CVD synthetic        diamond material may be identified, such modification including        changes to physical shape and annealing treatments such as those        which modify colour.

The exact function of the mark of origin or fingerprint typicallydetermines the form of the mark that is preferred.

In its simplest form the mark may merely comprise a substantial portionof the diamond layer, object or synthetic gemstone, or a single broadlayer within it, exhibiting an unnatural characteristic which is onlyobservable under specific artificial conditions of observation and doesnot significantly affect the colour of any object prepared from thelayer under normal illumination. The obviously artificial element to themark may result from its colour, possibly in combination with thespecific artificial conditions applied in order to observe the colour,or the geometry of the boundaries or distribution of the marked layerwithin the whole layer, primarily observed as the geometry with which itintercepts the boundaries of the layer, object or synthetic gemstone, orin the way it influences the appearance of an object of given geometrywhen viewed from one or more specific viewing angles.

In a more complex form, in order to generate a distinctive mark such asa trademark, the mark of origin or fingerprint generally comprises oneor more sets of characteristic layers, either distributed periodicallythroughout the diamond layer, object or synthetic gemstone or, in thecase of a single set of characteristic layers, placed in an appropriatelocation, generally not too near one of the edges of the object suchthat minimal removal of material will remove it, nor generally such thatthe mark is concealed and made difficult to observe by componentsessentially permanently attached to the object in normal use. In thecase of a synthetic gemstone, a single set of characteristic layers maybe located near the middle of the thickness of the cut stone, or ifbiased away from the middle then preferably biased in order to enhancethe intended benefit of the layer.

The ideal location of a layer within a gemstone is dictated by a numberof considerations:

-   -   a) The tagging layer should not be easily removable, and thus        not wholly close to an external surface such as the table or        culet.    -   b) The tagging layer should not provide visible colour to the        gemstone. The effect of the tagging layer on the colour of the        gemstone will depend on the intrinsic optical absorption        properties of the tagging layer material and the path length        within the layer of light rays reaching the eyes of the viewer.        The latter depends on position and thickness of the layer. It is        also a function of the cut of the stone, although        generalizations are possible.    -   c) The tagging layer should be positioned so that in any mount        not normally easily removed, such as a jewellery setting, the        volume of the layer can be efficiently excited by a deliberately        applied external light source used during identification, the        key point being that this light distribution may differ from        that in normal viewing conditions, being for example a high        intensity parallel beam rather than more diffuse.    -   d) The tagging layer should be positioned so that in any mount        not normally easily removed, such as a jewellery setting, a        significant proportion of the light emitted by the tagging layer        is made available to the viewer or other means of detection.        Whilst this may seem self-evident, the angle for total internal        reflection in diamond is only 22.4° away from normal incidence        due to its high refractive index and this gives some unexpected        results, discussed further below. Again this is a sensitive        function of the cut of the stone, but generalizations can be        made.    -   e) Aspects (b)-(d) interact, such that ideally the tagging        layer, or the majority volume of it, is placed at that position        in the final synthetic gemstone which is most effectively        illuminated and which most efficiently passes back the emitted        radiation to the viewer or detection system, but which does not        provide excessive enhancement of the effect on the visible        colour of the stone.

The effect of total internal reflection on the viewing of luminescencearising from the volume of a CVD diamond plate or stone will now beconsidered. As an example, consider a rectangular flat plate withprecise flat faces all of the type {100}. An external light beam ofgiven direction will pass into the diamond whatever its direction,refraction resulting in it being bent to much closer to the normal tothe interface at the interface. It may possibly be internally reflectedonce by a face parallel to a different axis but will then exit thesample again, essentially exiting after a single pass through thematerial. However, when luminescence arises in the volume of the stoneits direction of emission is generally equally distributed throughoutthe solid angle (although it is possible to identify defects with anon-uniform pattern of emission). Imagine then the 4π solid angle of theuniform irradiation field, interacting by total internal reflection withthe faces of the plate. Any radiation not within 22.40 of the normal ofone of the 3 face types ({100}, {010}, {001}) will be permanentlytotally internally reflected. Now imagine a small corner facet. Thiswill be able to emit all radiation internally incident on it within22.4° of its normal from the entire volume of the sample, very little ofthis radiation escaping through the main faces of the plate. Thus, inthis case, strong emission is observed from facets which are notparallel to the main facets. However, at each facet which is emittingthe emitted light is refracted to largely fill the hemisphere of solidangle centred on the normal to the plane of the facet. From this simpleexample it is clear that the behaviour of the excitation source passinginto a three dimensional diamond object and the emission of lightgenerated within. it can be distinctly different.

As a further example to put this into context, in a typical roundbrilliant cut synthetic diamond gemstone, a layer near the table isgenerally likely to affect the visible colour of the stone and berelatively easily excited by a specific source, but may not provide goodintensity in the emitted beam out through the table because all lightmore than 22.40° away from the normal to the table will be totallyinternally reflected, then possibly exiting the stone below the girdle.In contrast, a layer near the culet will generally impact less on thevisible colour of the stone, may require more careful control of theexcitation beam angle and distribution in order for it to be effectivelyexcited, but may be more effective in providing emission of luminescencethrough the table as a result of total internal reflection at thepavilion facets below the girdle towards the culet.

One particular type of layer is one in which the prime active impurityis nitrogen in the form of NV⁰ and NV⁻ centres that generate 575 nm and637 nm luminescence lines respectively, together with their associatedvibronic bands, under suitable shorter wavelength excitation. Thecombination of these emissions appears orange/red and is generallyreferred to as ‘orange luminescence’. Such luminescence extinguishesessentially instantaneously when the excitation source is removed.Whilst clearly visible under suitable artificial illuminationconditions, under normal viewing conditions and with appropriatelychosen defect concentrations and/or total defect numbers as envisaged inthis invention, this type of centre does not significantly alter theperceived colour of the gemstone.

Another particular type of layer is one in which the dominatingimpurities are boron and a suitable donor such as nitrogen.Donor-acceptor pair recombination may occur in such a layer and thelayer then exhibits characteristic blue luminescence over a broad bandpeaking in the range 400 nm to 500 nm, typically in the region of 500 nmunder suitable shorter wavelength excitation. Phosphorescence builds upand then saturates with time during the period under the excitationsource, and is visible as its intensity decays for a period of timeafter the removal of the excitation source, the time period typicallybeing several seconds although it can be as long as a minute or more. Inthis type of layer the nitrogen has two important roles: providing thedonor for donor/acceptor pair recombination and, by compensating theboron, reducing the B-related absorption which might otherwise causeobservable blue colour. With suitable artificial illumination conditionsdonor-acceptor luminescence and phosphorescence may be clearly visiblefor a layer that, under normal viewing conditions, does notsignificantly alter the perceived colour of the diamond object. Otherdonors, such as intrinsic defects, may contribute to this type ofluminescence and phosphorescence.

A particularly advantageous arrangement would be where these two typesof layer are both present, possibly used alternately or in some otherpattern, within unmarked material. Alternatively, one type of layer,preferably the orange luminescent layer, may fill substantially all thevolume not occupied by the other. Shorter wavelength excitation can thenbe selected to excite both the (575 nm/637 nm) orange luminescence andthe (500 nm) blue-band phosphorescence, or a suitable combination ofwavelengths can be used.

The method of viewing the mark is in part related to the excitationwavelength(s) used. Using sub-bandgap illumination (i.e. light withinsufficient energy to excite electrons right across the band gap indiamond, and thus not normally absorbed by diamond itself), theradiation would be transmitted through the volume of the stone, beingabsorbed only by the defects in the doped marker layers, enabling thevolume of the doped marker layers to be excited. Since the wavelengthsemitted from the layers are also transmitted by diamond (although someabsorption may occur in the defects in the layers) the person viewingthe stone would be able to see, for example by viewing through the tableof the stone, a substantial area of the layer which is emitting thecolour within the diamond volume.

On switching off the excitation source, since the orange luminescenceturns off with the excitation source, the perceived colour of the lightemitted from the CVD diamond material, or the tagging layers within it,would change from orange, or some orange/blue combination, to blue. Thiswill be termed the orange/blue flash. The visibility of such a change incolour, particularly the visibility of the orange component against theblue phosphorescence, may need to be enhanced by use of suitablefilters. Sub-bandgap illumination would be the preferred excitationwavelength for simple demonstration of the existence of the mark in thepiece of synthetic gemstone, allowing its detection through the table ofa cut stone even when the layers are significantly further down in thematerial. In some instances where the material is formed in the shape ofa synthetic gemstone it may be preferred to place these layers below thegirdle of the stone, so that the edges of the layers are generallyconcealed by the mount. In other instances it may be preferred to placethe layers above the girdle, allowing these edges to be viewed on thecrown facets. The closer to the centre of the synthetic stone the layersare positioned, the more difficult it is to remove the mark byrepolishing the stone without significant weight loss. The orange/blueflash is particularly characteristic of the combination of theseparticular marking layers. It provides a unique characteristic not seenin natural diamonds or likely to occur accidentally in synthetic stones.Those skilled in the art will understand that other colour combinationsmay be possible using other types of defects in the diamond material,and that the invention is not limited to any particular colours and orviewing conditions, but extends generally to any viewable distinctivemark not normally observed in natural diamond which does notsubstantially degrade the visual characteristics of the layer orgemstone under normal viewing conditions.

Using above-bandgap illumination (i.e. light with sufficient energy toexcite electrons right across the band gap in diamond), the diamondmaterial would itself directly absorb the incoming radiation and limitthe penetration depth to near the surface of the stone. This wouldpotentially limit the visibility of the layers to those areas close tothe surfaces of the stone which are directly exposed to the artificialillumination. Two effects may broaden the region in which theilluminating irradiation is absorbed or a response observed, the firstis where the excitation radiation is near-bandgap, so that theattenuation of the radiation in the diamond material is rising but isnot yet limiting the penetration to a few microns depth at the surface,and the second is where charge carriers excited at the surface by theincoming radiation are able to drift further into the diamond item andthen cause excitation of the layers further into the bulk. The abilityfor this to happen depends on a variety of factors including the generalpurity and crystal perfection of the synthetic diamond material outsidethe specifically marked layers.

However, the benefit of using above-bandgap radiation is generally tolimit the region excited to the surface of the diamond item and thusprovide greater definition of the pattern of the layers, althoughviewing this detail often requires more sophisticated viewing equipment,particularly in view of the thickness of the layers. In this domain,viewing only the exposed edges of the layers and considering thespecific example of using alternate orange luminescing and bluephosphorescing layers, both layers would be visible during excitationbut only the layers showing blue-band phosphorescence would be visibleafter the excitation ceased. Above-bandgap illumination is particularlyuseful in a) enabling the detailed geometry of a mark intended as atrademark to be observed, and b) where the geometry of the layers isused to emphasise the synthetic nature of the material where the colouror colours, spatially or temporally separated, alone may not besufficient.

Where a single marking layer is used, this may occupy a substantialvolume of the whole layer, object or synthetic gemstone. Where one ormore layers are used in a single group or pattern then the lower boundof the thickness of each of the layers would typically exceed 10 μm,more typically exceed 20 μm, even more typically exceed 50 μm, even moretypically exceed 100 μm, and most typically exceed 200 μm, and the upperbound of the thickness of each of the layers would typically be lessthan 1000 μm, more typically be less than 600 μm, even more typically beless than 400 μm and most typically be less than 250 μm, the basiccriteria being that for the concentration of impurities used and theassociated absorption and luminescence characteristics, the layers arethin enough so as not to significantly colour the cut stone when viewedunder normal light, whilst being sufficiently thick to providesufficient visibility under the selected excitation wavelengths. Anadditional requirement may be for the layers to be thick enough fortheir geometry to be precisely measurable, for example using an aboveband gap viewer as described earlier. A further combination is where onemarking layer or characteristic feature extends throughout the majorityor the whole of the diamond object except where a second layer is formedwithin this region.

Where one or more layers are used in multiple or a repeating patternspread through the volume of the stone then the individual layers may bethinner, a key parameter being the total thickness of all the layers ofthe same type. In such circumstances the lower bound of the thickness ofeach of the layers would typically exceed 2 μm, more typically exceed 5μm, and most typically exceed 10 μm, and the upper bound of thethickness of the layers would typically be less than 100 μm, moretypically be less than 50 μm, even more typically be less than 20 μm.

In particular, excluding for the moment the geometrical issues describedearlier arising from the effect of total internal reflection, viewingnear normal to the plane of the marked layers using sub-bandgapradiation, the critical parameter would be the concentration of emittingcentres through the thickness of the layer projected onto the plane ofthe layer. That is, the observed brightness of the layer would beproportional to the product of the intensity of the exciting radiation,the concentration per unit volume of the irradiating centre, and thethickness of the layer. Other effects can also contribute to theobserved intensity, including the effect of self absorption elsewhere inthe material. Thus thicker layers would be preferred whereconcentrations of dopants were very low. Such conditions may be favouredby the requirement to have minimal impact on the overall growth process.

In contrast, viewing the edges of the layers in above-bandgapexcitation, the depth of material sampled would be largely fixed by theradiation wavelength and thus the observed brightness would beproportional to the product of the intensity of the exciting radiationand the concentration per unit volume of the relevant defect centres,with the thickness of the layer determining the lateral extent of thearea emitting. Thicker layers may again assist in improving visibilitywhere concentrations of dopants are low, by increasing the area to beobserved.

A third case is when viewing near normal to the plane of the markedlayers using sub-bandgap radiation in a cut gemstone. This is describedin more detail later, where total internal reflection on the lowerfacets dominates the behaviour. Here, although the viewing direction isapparently normal to the plane of the layer, the beam actually beingobserved is largely parallel to the layer until total internalreflection occurs, so that the critical parameter would be theconcentration of emitting centres through the thickness of the layerprojected onto the plane of the layer. That is, the observed brightnessof the layer would be proportional to the product of the intensity ofthe exciting radiation, the concentration per unit volume of theirradiating centre, and the lateral extent of the layer. Other effectscan also contribute to the observed intensity, including the effect ofself absorption elsewhere in the material.

Useful concentrations of the various impurities have been evaluated andfound to be as detailed below. However, those skilled in the art willunderstand that there is considerable interaction between the type ofgrowth process used, the concentration of other impurity centres orstructural defects which may for example result in quenching ofluminescence or a change in the charge state and thus the opticalbehaviour of the impurities used for the tagging layers. In addition, itis well known that the uptake of impurities varies with the specificgrowth sector of diamond involved, for example the {111} growth sectoroften taking up higher concentrations of impurities than the {100}growth sector. For simplicity the data given below relates to the {100}growth sector and requires suitable modification where other growthsectors are involved.

Thus, using a microwave process such as that detailed in Example 1, ithas been found that for generating the 575 nm luminescence at levelssuitable for tagging, whilst reducing the effect on colour and visibleabsorption to acceptable levels, the optimum value of the molecularnitrogen concentration in the gas phase lies in the range having anupper limit of preferably 10 ppm, more preferably of 3 ppm, even morepreferably of 1 ppm, even more preferably of 0.5 ppm and most preferablyof 0.2 ppm, and a lower limit of preferably 0.01 ppm, more preferably0.05 ppm, and most preferably of 0.1 ppm. In terms of the nitrogenincorporated into the solid of the material, this is not always easy tocharacterize in diamond at these low levels but is generally measured interms of atomic fraction to be about 10³-10⁴ lower than that of themolecular concentration in the gas phase. Molecular nitrogen is not theonly useful source of nitrogen, for example NH₃ would also be of use,although the relative activation/incorporation of the N may then bedifferent.

Likewise, using a microwave process such as that detailed in Example 1,it has been found that for generating B/N donor acceptor pairphosphorescence at levels suitable for tagging, whilst reducing theeffect on colour and visible absorption to acceptable levels, theconcentration of both boron and nitrogen needs to be controlled. Inparticular, the concentrations of B and N in the solid are preferablywithin a factor of 30, and more preferably within a factor of 10, evenmore preferably within a factor of 5, even more preferably within afactor of 3, and most preferably within a factor of 2, preferably ineach case with the concentration of the boron exceeding theconcentration of the nitrogen. One benefit of this is that the nitrogencompensation of the boron reduces the effect that the boron has on thecolour of the material. A further limit is provided by evaluating theeffect of the boron on the visible colour. Experimentally it has beendetermined that uncompensated boron provides a visually detectable bluecolour when present in a round brilliant as a layer where the product ofthe thickness of the layer and the uncompensated boron concentration inthat layer exceeds 0.1 ppm.mm (e.g. 1 mm thickness of 0.1 ppmuncompensated boron or similar). Phosphorescence however can begenerated in layers with much lower levels of boron, with suitablelevels of phosphorescence having been observed in 200-400 μm thicklayers with concentrations of boron in the solid of 0.01 ppm to 0.001ppm, with the indication that even lower levels may be sufficient.

It has been determined experimentally that the phosphorescence intensityarising from boron/nitrogen donor acceptor pair recombination can bereasonably approximated as the sum of two second-order decays, each witha characteristic time constant. The form of this is given below:I/I ₀ =A(1+t/τ ₁)²+(1−A)/(1+t/τ ₂)²

It should not be assumed from the observation that the data can befitted to an equation of this form that there are two, or only two,distinct types of centre present. In some cases there is only one timeconstant with a significant amplitude. However, it is generally foundthat where two time constants can be found, they differ by a factor ofabout 8 to 10. At higher concentrations of boron, the longer timeconstant is generally still relatively short, typically under 2 secondsand often under 1 second. At lower concentrations the slower decayingcomponent generally becomes more dominant and its time constantincreases to greater than 3 seconds. This has three related advantages:

-   -   a) the integrated pumping period effectively increases in        proportion to the decay time constant (phosphorescent decay        observed just after removing the excitation beam could have been        excited a longer time ago);    -   b) the intensity at any given time after excitation as a        consequence increases; and    -   c) typically for optimum detection by eye the intensity needs to        be visible for at least 2-3 seconds.

The relationship between boron concentration and the value of the longertime constant is not fully determined, but under test conditions used,for example as in Example 1, boron concentrations below 0.1 ppm in thesolid seem particularly beneficial.

Thus the preferred concentration of boron in the solid lies in theconcentration range with an upper bound of 1 ppm, more preferably 0.3ppm, even more preferably 0.1 ppm, even more preferably 0.05 ppm andmost preferably 0.02 ppm, and with a lower bound of 0.0001 ppm, morepreferably 0.0003 ppm, even more preferably 0.001 ppm, even morepreferably 0.002 ppm, and most preferably 0.005 ppm. The incorporationratio of the boron is typically such that the preferred values for themolecular concentration of diborane in the gas phase are a factor of 10higher than these values.

As is generally well known, the incorporation ratio for nitrogen is muchlower than that for boron. As a consequence, whilst the optimumconcentration in the solid may be close to but below that of the boronfor enhanced phosphorescence, the concentration in the gas phase isgenerally much higher. Typically the concentration range for thenitrogen is chosen to meet the other criteria—that is the boronconcentration in the solid is first set and then the relativeconcentration of the nitrogen in the solid set, thus largely determiningthe concentration of nitrogen to add to the gas phase dependent on theexact incorporation ratio achieved under the growth conditions in use.However, preferred values for the molecular nitrogen concentration inthe gas phase for the production of the blue phosphorescent layer lie inthe range bounded by an upper limit of preferably 50 ppm, morepreferably 20 ppm, even more preferably 10 ppm, even more preferably 5ppm and most preferably 2 ppm, and a lower limit of preferably 0.02 ppm,more preferably 0.05 ppm, even more preferably 0.1 ppm, even morepreferably 0.2 ppm and most preferably 0.5 ppm. Again, molecularnitrogen is not the only useful source of nitrogen, for example NH₃would also be of use, although the relative activation/incorporation ofthe N may then be different.

In the case of the Si-related 737 nm centre, the nature and behaviour ofthis defect is less well understood at this time, however again very lowconcentrations of silicon, of the order of 10 ppm to 0.0001 ppm, arebelieved to be suitable to generate the defect in sufficient quantity,provided the other necessary components are present. One particularissue is thought to be the charge state of the defect; in the presenceof boron the charge state may change from the state needed for 737 nmluminescence, whilst the presence of nitrogen may help to stabilize thedefect in the correct charge state. Models for the defect include asubstitutional Si with an adjacent vacancy in the neutral charge state,a silicon vacancy complex, and two substitutional silicon atoms alongthe <111> axis bound with vacancies. The total integrated intensity ofluminescence from the Si-related centre is typically much less than forthe NV⁰ and donor-acceptor luminescence and phosphorescence. Inaddition, it lies in a region of the spectrum (737 nm) where the eye isless sensitive. Consequently the primary methods for its detectioninvolve spectroscopic instruments rather than direct viewing. Detailedlayer structures in the incorporation of the Si-related centre are lessadvantageous because they are harder to view or identify in other ways.That said, the 737 nm line can be viewed in systems using, for example,electronically enhanced imaging, particularly in combination with theuse of suitable filters, and layer structures can also be detected bymeans such as confocal luminescence depth profiling over a limitedwavelength region or combined with spectroscopic analysis. The 737 nm Sirelated centre has a complex set of excited states and can thus beexcited by a range of different laser wavelengths including 488 nm and514 nm, and is particularly efficiently excited by the 633 nm HeNelaser. However, other light sources with wavelengths in the range480-700 nm would be suitable.

Suitable sub-bandgap wavelengths for viewing the marks or tagging layerscan be determined as follows. Orange luminescence (from the 575 nm and637 nm optical centres) can be excited by a range of wavelengths such as514 nm, 488 nm and shorter wavelengths, but excitation efficiency isreduced as wavelengths approach the UV and it is worth noting that the637 nm centre is not excited by wavelengths below about 400 nm. Incontrast, blue-band phosphorescence is more efficiently excited bywavelengths approaching the short UV, such as the mercury line at 254nm. These trends in the efficiency of excitation are not particularlylimiting, however, and a range of wavelengths can be used to excite bothemissions sufficiently well.

Suitable above-bandgap radiation for viewing one or more tagging layersin the near-surface region would be at 193 nm and shorter wavelengths.Generally high luminance sources can be used and good spatial resolutionstill obtained because of the strong attenuation in the diamond. Aninstrument particularly suited to precise viewing and characterisationof the layers using above bandgap UV radiation is the ‘DiamondView™’instrument, developed by the Diamond Trading Company which combines asuitable UV source with digital image capture and allows the study ofboth luminescence and phosphorescence in diamond samples even down torelatively low luminosity levels.

The optical characteristics when viewed using suitable conditions, orthe spatial distribution of those regions providing the opticalcharacteristics, or the combination of these two, provide a distinctionfrom other forms of diamond such as natural diamond or CVD diamond knownin the art. In this respect, whilst phosphorescence in natural bluediamonds is known, and orange luminescence, although relatively rare, isalso known in natural stones, they are not known to exist in the samenatural stone, nor are they known in the form of clearly defined layersin natural stones and there is no known example of natural diamond thatshows the 737 nm silicon-related line.

It has been determined that the orange/blue flash effect may best beobserved by exciting and identifying the orange 575 nm and blueluminescence/phosphorescence bands separately. The rationale for this isnow discussed.

When a phosphorescent centre is present and excited by a suitableexcitation wavelength, the phosphorescent centre is not only visibleafter the illumination is removed, but also whilst the illumination ison. This apparent luminescence from the phosphorescent centre will bestronger than any subsequent phosphorescence after the excitation sourceis removed, by a degree dependent on the lifetime of the centre and thetime of measurement. Consequently, even a relatively weak phosphorescentcentre can result in significant luminescence whilst the source is on.

Considering the use of a single wavelength or band of wavelengths toexcite both orange luminescence and blue PL/phosphorescence, it has beennoted that if the blue luminescence band is present in considerablestrength, then this can make simultaneous observation of the 575 nm bandextremely difficult. If an attempt is made to block the blue PL andobserve the 575 nm region through a suitable filter (e.g OG550) then afalse impression of orange ‘575 nm PL’ would be observed as the longwavelength tail of the blue luminescence would also be transmitted bythe filter. There may also be additional confusion, as when theexcitation source is switched off the long wavelength tail of the bluephosphorescence band would be observed through the filter.

Thus, the test for orange 575 nm luminescence should ideally bedetermined first followed by the test for blue phosphorescence. It isinadvisable to excite the blue phosphorescence first as this may take upto a minute to decay to a level where 575 nm luminescence may then beexcited and observed. 575 nm luminescence may be excited withwavelengths in the range 225 nm to 575 nm but may only be excitedwithout also stimulating blue luminescence/phosphorescence in the range300 nm to 575 nm. The strength of the 575 nm emission depends on the CVDsynthetic having a suitably high concentration of 575 nm centres in thetagging layer and/or a sufficiently thick layer. Wavelengths ofexcitation greater than about 380 nm are within the spectral response ofthe eye. This could severely affect the observation of the 575 nm band.In this case, a suitable filter is required to substantially orcompletely block the excitation source from reaching the eye of theperson who is viewing the 575 nm band.

The test for 575 nm luminescence is then followed by the test for bluephosphorescence. The 575 nm excitation source and the viewing filtershould be removed. Shortwave ultraviolet excitation at a wavelength orwithin a wavelength band in the range 225 nm to about 254 nm should thenbe switched on to excite any blue luminescence. The 575 nm band willalso be stimulated by the shortwave excitation, but will almostcertainly be dominated by blue PL. After several seconds the shortwaveexcitation should be switched off and blue phosphorescence observed.There will be no contribution from the 575 nm centre which does not showphosphorescence. If by using this sequential excitation method theorange/blue flash is observed then the stone under test is a tagged CVDsynthetic with the specific structure described earlier.

A particularly advantageous form of inexpensive viewer for generaldetection of the presence of the mark in this invention would combine asmall box to fit over the CVD diamond layer, object or syntheticgemstone to exclude ambient light, with an excitation light sourceentering the box and a viewing window, possibly in the form of amagnifying lens, with a filter to remove the excitation wavelength.Alternatively ambient white light could also be removed by filtersrather than excluded from the viewing box, with the filters then beingessentially band-pass filters for the orange luminescence and blue-bandphosphorescence.

Sub-bandgap illumination would be the preferred excitation method forthe simple demonstration of the existence of tagging layers/marks in thepiece of synthetic diamond material for example by the orange/blue flasheffect.

Sub-bandgap illumination would penetrate the whole volume of thesynthetic stone and therefore permit excitation of tagging layers at anylocation within it. Tests have shown that in general the whole volume ofthe stone is equally illuminated, even using an excitation beamintroduced from one side only, since the resultant image of theluminescence is not sensitive to the position/direction of theexcitation beam. This method of excitation allows detection of theluminescence through the table of a cut stone even when the layers aresignificantly buried within the material and from a cut stone in arub-over jewellery setting.

As stated above, the step of introducing a dopant into the source gas ina controlled manner also leads to the generation of diamond of highcolour. For example, the method of the present invention is able toprovide a CVD diamond layer having high colour where the synthesisatmosphere comprises a gas having a first impurity atom type that wouldprevent high colour diamond from being produced. All of the embodimentsdescribed above are equally applicable to the generation of diamond ofhigh colour.

Typically, the introduction of boron or silicon to the growth gases cansignificantly increase the threshold nitrogen concentration to levelsthat might be present in growth environments when relatively littleattention is given to nitrogen elimination. This allows diamond to begrown in the presence of relatively high concentrations of nitrogenwithout the degradation of the optical and other properties that wouldotherwise result because of the incorporation of defects such as vacancyclusters and hydrogen-related defects. In addition it has beensurprisingly found that this is possible even though there may besignificant incorporation into the grown diamond of both nitrogen andthe second impurity atom type.

We define the term “high colour” in this invention in two different waysdepending upon the form of the diamond material and the application towhich it is put. The definition of “high colour” used herein is thatwhich is most applicable to the form of the diamond layer produced andits application. When the diamond is in the form of a round brilliant(i.e. when the diamond is in the form of a gem stone), the GIA colourscale is generally used. When the diamond is in the form of a plate,etc, to be used in a technological application, the material isgenerally defined in terms of its absorption characteristics. Absorptioncharacteristics are also used to define polycrystalline diamond.

Thus, when the diamond layer of the invention is in the form of a gemstone, ‘High colour’ is generally defined as being colour better than Kon the Gemological Institute of America (GIA) gem diamond colour scaleas determined for a 0.5 ct round brilliant, and defines material withabsorption close to the theoretical limit for impurity free diamond.Herein, high colour equates to colours equivalent to the natural diamondcolour grades of D to better than K, where these are colour grades onthe Gemological Institute of America (GIA) colour scale, see ‘DiamondGrading ABC’, V. Pagel-Theisen, 9th Edition, 2001, page 61). This colourscale, which is the most widely used and understood diamond colourscale, is shown in Table 1. Table 1 is derived from ‘Diamond GradingABC, The Manual’, Verena Pagel-Theisen, 9^(th) Edition 2001, publishedby Rubin and Son n.v. Antwerp, Belgium, page 61. The colours aredetermined by comparison with standards. The determination of the colourof diamonds is a subjective process and can only reliably be undertakenby persons skilled in the art. TABLE 1 Colour on GIA Impression of ScaleColour D Colourless E F* G Almost H colourless I J K L Pale yellowish MN Very light O yellowish P Light yellow Q R Yellowish S T U V W X Y Z Z+Fancy Colours*colourless for round brilliants less than 0.47 cts.

The clarity scale of the Gemological Institute of America (GIA), whichis the most widely used clarity scale, is shown in Table 2. Table 2 isderived from ‘Diamond Grading ABC, The Manual’, Verena Pagel-Theisen,9^(th) Edition 2001, published by Rubin and Son n.v. Antwerp, Belgium,page 61. It takes into account both internal and external flaws on a cutdiamond. Typically, examination is made with the aid of a 10× magnifieror loupe by an experienced grader with appropriate illumination for thetype of defect that is being sought. TABLE 2 Description DesignationNotes Flawless FL Flawless: No internal or external features, with theexception of extra facets that are not visible from the upper facet;naturals at the girdle which neither widen it nor make it irregular;non-reflecting internal growth lines which are neither coloured norwhite and do not affect transparency. Internally IF Loupe Clean(internally flawless): no inclusions and only flawless minor externalfeatures, with the exception of small external growth lines. Very veryVVS 1 & 2 Very, very small inclusions 1 & 2: very small inclusionsslightly which are difficult to see; in the case of VVS1, these areincluded very difficult to see and then only from the lower facet orthey are so small and sufficiently near the surface to be easily cutaway (potentially flawless). In the case of VVS2, the inclusions arestill very difficult to see. Typical inclusions include occasionalspots, diffuse, very fine clouds, slight beading on the girdle, internalgrowth lines and very small fissures, nicks or blow indentations. Veryslightly VS 1 & 2 Very small inclusions 1 & 2: smaller inclusionsranging included from those which are difficult to see to those whichare somewhat easier to see. Typical inclusions are small includedcrystals and small fissures, more distinct small clouds and groups ofdot-like inclusions. Slightly SI 1 &2 Small inclusions 1 & 2: inclusionswhich are easy (SI1) included or very easy (SI2) to see; the inclusionsare often in a central position, can be recognised immediately and insome cases are also visible to the naked eye. Imperfect I 1 to 3Inclusions 1, Inclusions 2 and Inclusions 3: distinct inclusions whichin most cases are easily visible to the naked eye through the crown; inthe case of inclusions 3, stone durability can be endangered. Typicalinclusions are large included crystals and cracks.By “high clarity” is meant herein a clarity of at least SI 1 as definedin Table 2, preferably at least VS 2.

The GIA diamond gem grading system is the most widely used grading scalefor diamond gems and generally considered the definitive grading scale.For the purposes of this application all gem colour grades are based onthe GIA colour grades, and other gem properties such as clarity arelikewise based on the GIA grading system. For a given quality ofdiamond, i.e. material with given absorption characteristics, the colourof a gem also varies with the size and cut of gem produced, moving topoorer colours (to colours towards Z in the alphabet) as the stone getslarger. To enable the colour system to be applied as a material propertyit is thus necessary to further fix the size and type of cut of thegemstone. All GIA colour grades given in this specification are for astandardised 0.5 ct round brilliant cut unless other stated.

Such colour grades are perceived by a skilled diamond grader as beingnearly colourless or colourless. The diamond of the invention may havecolour better than J, preferably better than I, preferably better thanH, preferably better than G, preferably better than F, or preferablybetter than E. The diamond layer of the invention has “very high colour”where the colour is D to F on the GIA gem diamond colour scale asdetermined for a 0.5 ct round brilliant.

For technological applications and for polycrystalline diamond layers ofthe present invention, “high colour” is generally defined as themajority volume of the material having a particular absorptioncoefficient at certain specific wavelengths in the near ultraviolet andvisible part of the electromagnetic spectrum (that is wavelengths in therange approximately 270 nm to 800 nm) when measured at room temperature.In particular, the absorption coefficients of the diamond at thewavelengths 270 nm, 350 nm, and 500 nm are of particular relevance,being generated by the defects arising from common impurities undernormal process conditions and often playing a significant role on thecolour of the material or the absorptions which may limit its use. Thusthe CVD diamond material of the invention described above will alsopreferably have one, more preferably two, more preferably three, andmost preferably all of the following characteristics (i), (ii), (iii),(iv), observable in the optical absorption spectrum:

-   -   i) an absorption coefficient measured at room temperature at all        wavelengths between 300 and 1000 nm which is less than 2 cm⁻¹,        more preferably less than 1 cm⁻¹, even more preferably less than        0.5 cm⁻¹, and most preferably less than 0.2 cm⁻¹;    -   ii) an absorption coefficient at 270 nm which is less than 2        cm⁻¹, more preferably less than 1 cm⁻¹, even more preferably        less than 0.5 cm⁻¹, and most preferably less than 0.2 cm⁻¹;    -   iii) an absorption coefficient at 350 nm which is less than 1.5        cm⁻¹, more preferably less than 0.75 cm⁻¹, even more preferably        less than 0.3 cm⁻¹, and most preferably less than 0.15 cm⁻¹;    -   iv) an absorption coefficient at 520 nm which is less than 1        cm⁻¹, more preferably less than 0.5 cm⁻¹, even more preferably        less than 0.2 cm⁻¹, and most preferably less than 0.1 cm⁻¹.

Preferably the majority volume of the layer is formed from a singlegrowth sector.

Material of the invention can have sharp absorption features in therange 720-750 nm, but these contribute little to the colour and are thusnot restricted by these definitions.

To derive the absorption coefficient the reflection loss must first besubtracted from the measured absorbance spectrum. When subtracting thereflection loss, it is important to take account of the spectraldependence of the reflection coefficient. This can be derived from thewavelength dependence of the refractive index of diamond given by F.Peter in Z. Phys. 15, 358-368 (1923). Using this and standard formulaefor the dependence of reflection loss for a parallel-sided plate on therefractive index, the effect of reflection losses on the apparentabsorbance can be calculated as a function of wavelength and subtractedfrom measured spectra to allow absorption coefficient spectra to becalculated more accurately.

Alternatively, “high colour” may be defined using the CIELAB coloursystem. This colour modelling system allows colour grades to bedetermined from absorption spectra. This system takes into account thethree visual attributes to colour: hue, lightness and saturation. Hue isthe attribute of colour that allows it to be classified as red, green,blue, yellow, black or white, or a hue that is intermediate betweenadjacent pairs or triplets of these basic hues (Stephen C. Hofer,Collecting and Classifying Coloured Diamonds, 1998, Ashland Press, NewYork). White, grey and black objects are differentiated on a lightnessscale of light to dark. Lightness is the attribute of colour that isdefined by the degree of similarity with a neutral achromatic scalestarting with white and progressing through darker levels of grey andending with black.

Saturation is the attribute of colour that is defined by the degree ofdifference from an achromatic colour of the same lightness. It is also adescriptive term corresponding to the strength of a colour. The diamondtrade uses adjectives such as intense, strong and vivid to denotedifferent degrees of saturation assessed visually. In the CIELAB coloursystem, saturation is the degree of departure from the neutral colouraxis (defined by saturation=[(a*)²+(b*)²]^(1/2), see hereinafter).Lightness is a visual quality perceived separately from saturation.

In cases where material with particular absorption properties has beengrown to a limited thickness, it is useful to be able to predict, fromabsorption spectroscopy measurements carried out on a thinparallel-sided plate of the material, what colour a round brilliantwould be if it were polished from a thicker slab of uniform materialwith the same absorption coefficient spectrum. A simple routine fordoing this is described here. The first stage of this routine is thederivation of CIELAB chromaticity coordinates for a parallel sided plateof material from its measured transmittance in the visible region of thespectrum.

The perceived colour of an object depends on thetransmittance/absorbance spectrum of the object, the spectral powerdistribution of the illumination source and the response curves of theobserver's eyes. The CIELAB chromaticity coordinates quoted in thisspecification have been derived in the way described below.

Using a standard D65 illumination spectrum and standard (red, green andblue) response curves of the eye (G. Wyszecki and W. S. Stiles, JohnWiley, New York-London-Sydney, 1967) CIE L*a*b* chromaticity coordinatesof a parallel-sided plate of diamond have been derived from itstransmittance spectrum (between 350 nm and 800 nm with a 1 nm datainterval) using the relationships below.

-   S_(λ)=transmittance at wavelength _(λ)-   L_(λ)=spectral power distribution of the illumination-   x_(λ)=red response function of the eye-   y_(λ)=green response function of the eye-   z_(λ)=blue response function of the eye    X=Σ _(λ) [S _(λ) x _(λ) L _(λ) ]/Y ₀    Y=Σ _(λ) [S _(λ) y _(λ) L _(λ) ]/Y ₀    Z=Σ _(λ) [S _(λ) z _(λ) L _(λ) ]/Y ₀-   Where Y₀=□_(□)y_(□)L_(□)    L*=116(Y/Y ₀)^(1/3)−16=Lightness (for Y/Y ₀>0.008856)    a*=500[(X/X ₀)^(1/3)−(Y/Y ₀)^(1/3)] (for X/X ₀>0.008856, Y/Y    ₀>0.008856)    b*=200[(Y/Y ₀)^(1/3)−(Z/Z ₀)^(1/3)] (for Z/Z ₀>0.008856)    C*=(a* ² +b* ²)^(1/2)=saturation    h _(ab)=arctan(b*/a*)=hue angle

Modified versions of these equations must be used outside the limits ofY/Y₀, X/X₀ and Z/Z₀. The modified versions are given in a technicalreport prepared by the Commission Internationale de L'Eclairage(Colorimetry (1986)).

It is normal to plot a* and b* coordinates on a graph with a*corresponding to the x axis and b* corresponding to the y axis. Positivea* and b* values correspond respectively to red and yellow components tothe hue. Negative a* and b* values correspond respectively to green andblue components. The positive quadrant of the graph then covers huesranging from yellow through orange to red, with saturations (C*) givenby the distance from the origin.

It is possible to predict how the a*b* coordinates of diamond with agiven absorption coefficient spectrum will change as the optical pathlength is varied. In order to do this, the reflection loss must first besubtracted from the measured absorbance spectrum. The absorbance is thenscaled to allow for a different path length and then the reflection lossis added back on. The absorbance spectrum can then be converted to atransmittance spectrum which is used to derive the CIELAB coordinatesfor the new thickness. In this way the dependence of the hue, saturationand lightness on optical path length can be modelled to give anunderstanding of how the colour of diamond with given absorptionproperties per unit thickness will depend on the optical path length.

Much CVD material is brown because of a gradual rise in absorptioncoefficient towards shorter wavelengths. CVD synthetic round brilliantshave generally been produced from homoepitaxial CVD material with anorientation such that the table of the polished stone is parallel to theinterface with the diamond substrate on which CVD material wasdeposited. After substrate removal and polishing of the top and bottomfaces of the resulting slab, absorbance/transmittance spectra have beencollected and saturation values determined in the way described above.On polishing round brilliants with the depth limited by the thickness ofsuch slabs (a ‘depth-limited round brilliant’), an approximately linearrelationship has been found between the modelled saturation for theparallel-sided slab and the numerical colour grade of the resultingfinished stone, derived from GIA grades judged by a trained diamondgrader using the following transformation: D=0, E=1, F=2, G=3, H=4 etc.For moderate to weak saturations, the empirical relationship betweennumerical colour grade of brown/brownish depth-limited CVD roundbrilliants and the saturation (C*) modelled from theabsorbance/transmittance spectrum of the slab has been found to obey thefollowing approximate relationship:Round brilliant numerical colour grade=2×C*.

The observed linearity is supported by the following argument. Colourmodelling work has indicated that, for material with given absorptionproperties, for low to moderate saturations, there is an approximatelylinear relationship between the path length and C* values derived fromabsorbance/transmittance spectra using the routine outlined above. Withgiven viewing and lighting conditions, the average path length for lightreaching a viewer's eye from a round brilliant should be proportional tothe linear dimensions of the stone. It follows from that there should bean approximately linear relationship between the saturation for aparallel-sided slab and the saturation for a depth-limited roundbrilliant produced from the slab. Previous work has suggested that thereis an approximately linear relationship between the colour grade of apolished stone and its saturation. Taken together, this suggests thatthere should be an approximately linear relationship between the colourgrade of a depth limited round brilliant and the saturation derived fromthe absorbance/transmittance spectrum of the parallel-sided slab fromwhich it was polished.

From the discussion above it should be clear that where a relativelythin plate is produced, it is possible to predict, fromabsorbance/transmittance spectra measured for the plate, what colour around brilliant would be if it were polished from a thicker slab ofuniform material with the same absorption coefficient spectrum. In orderto do this, the reflection loss must first be subtracted from themeasured absorbance spectrum. The absorbance is then scaled to allow fora different path length and then the reflection loss is added back on.The absorbance spectrum can then be converted to a transmittancespectrum which is used to derive the CIELAB coordinates for the newthickness (for example, approximately 3.2 mm for a 0.5 ct roundbrilliant or 3.8 mm for a 1 ct round brilliant). When subtracting thereflection loss, it is important to take account of the spectraldependence of the reflection coefficient. This can be derived from thewavelength dependence of the refractive index of diamond given by F.Peter in Z. Phys. 15, 358-368 (1923). Using this and standard formulaefor the dependence of reflection loss for a parallel-sided plate on therefractive index, the effect of reflection losses on the apparentabsorbance can be calculated as a function of wavelength and subtractedfrom measured spectra to allow absorption coefficient spectra to becalculated more accurately.

The CVD diamond material of the invention described above will alsopreferably have one, more preferably two, more preferably three, morepreferably four, and most preferably all of the followingcharacteristics (1), (2), (3), (4), (5) observable in the majorityvolume of the layer, where the majority volume comprises at least 50%,preferably at least 55%, preferably at least 60%, preferably at least70%, preferably at least 80%, preferably at least 90% and mostpreferably at least 95% of the whole volume of the layer:

-   -   1) a charge collection distance of less than 150 μm, preferably        less than 100 μm, more preferably less than 50 μm, even more        preferably less than 20 μm, even more preferably less than 10        μm, and most preferably less than 5 μm, all charge collection        distances being measured at an applied field of 1 V/μm and at        300 K. Alternatively, a charge collection distance of greater        than 100 μm, preferably greater than 150 μm, preferably greater        than 200 μm, preferably greater than 300 μm, preferably greater        than 500 μm, preferably greater than 1000 μm. (Although most        applications benefit from a high charge collection distance,        some applications requiring very high speed detectors benefit        from low charge collection distances, particularly in        combination with the high crystal quality obtainable using a        preferred method of the invention);    -   2) a level of at least one impurity (excluding hydrogen) greater        than 0.05 ppm, more preferably greater than 0.1 ppm, more        preferably greater than 0.2 ppm, more preferably greater than        0.5 ppm, more preferably greater than 1 ppm, more preferably        greater than 2 ppm, even more preferably greater than 5 ppm, and        most preferably greater than 10 ppm. (Impurity concentrations        can for example be measured by secondary ion mass spectroscopy        (SIMS), glow discharge mass spectroscopy (GDMS) or combustion        mass spectroscopy (CMS), electron paramagnetic resonance (EPR)        and IR (infrared) absorption. In addition the uncompensated        single substitutional nitrogen concentration can be determined        from the peak of the absorption feature at 270 nm after baseline        subtraction (calibrated against standard values obtained from        samples destructively analysed by combustion analysis));    -   3) a total impurity concentration (excluding hydrogen) of        greater than 0.2 ppm, more preferably greater than 0.5 ppm, even        more preferably greater than 1 ppm, even more preferably greater        than 2 ppm, even more preferably greater than 5 ppm, even more        preferably greater than 10 ppm, and most preferably greater than        20 ppm. (Impurity concentrations may be measured as above);    -   4.1) weak free exciton luminescence in the cathodoluminescence        spectrum measured at 77 K, with the integrated intensity of the        free exciton luminescence preferably not exceeding 0.5, more        preferably not exceeding 0.2, even more preferably not exceeding        0.1, and most preferably not exceeding 0.05 of the integrated        free exciton luminescence intensity for a homoepitaxial CVD        diamond sample grown under high purity conditions, for example        those revealed in WO 01/96634. Alternatively, a strong free        exciton luminescence in the cathodoluminescence spectrum        measured at 77 K, with the integrated intensity of the free        exciton luminescence preferably exceeding 0.5, preferably        exceeding 0.6, preferably exceeding 0.7, preferably exceeding        0.8, preferably exceeding 0.9 of the integrated free exciton        luminescence intensity for a homoepitaxial CVD diamond sample        grown under high purity conditions; or    -   4.2) the strength of free exciton emission excited by 193 nm ArF        excimer laser at room temperature is such that the quantum yield        for free exciton emission is less than 10⁻⁴, more preferably        less than 10⁻⁵, and more preferably less than 10⁻⁶;        Alternatively, the free exciton emission is greater than 10⁻⁶,        preferably greater than 10⁻⁵, preferably greater than 10⁻⁴. Free        exciton emission can also be excited by above-bandgap radiation,        for example by 193 nm radiation from an ArF excimer laser. The        presence of strong free exciton emission in the        photoluminescence spectrum excited in this way indicates the        substantial absence of dislocations and impurities;    -   5) in EPR, a spin density which at g=2.0028 exceeds 1×10¹⁶ atoms        cm⁻³, more preferably exceeds 2×10¹⁶ atoms cm⁻³, more preferably        exceeds 5×10¹⁶ atoms cm⁻³, more preferably exceeds 1×10¹⁷ atoms        cm⁻³, more preferably exceeds 2×10¹⁷ atoms cm⁻³, and most        preferably exceeds 5×10¹⁷ atoms cm⁻³. Alternatively, the spin        density is preferably less than 5×10¹⁷ atoms cm⁻³, preferably        less than 2×10¹⁷ atoms cm⁻³, preferably less than 1×10¹⁷ atoms        cm⁻³, preferably less than 5×10¹⁶ atoms cm⁻³, preferably less        than 2×10¹⁶ atoms cm⁻³, preferably less than 1×10¹⁶ atoms cm⁻³.        (In single crystal diamond this line is related to lattice        defect concentrations and is typically large in poor quality        homoepitaxial diamond but small in high colour CVD diamond        formed using a high purity growth process).

Accordingly, this aspect of the invention provides a method of producinga CVD diamond layer having high colour comprising:

-   (i) providing a substrate;-   (ii) providing a CVD synthesis atmosphere in which there exists a    first gas comprising a first impurity atom type which has a    detrimental effect on the colour of the produced diamond layer; and-   (iii) adding into the synthesis atmosphere a second gas comprising a    second impurity atom type,    wherein the first and second impurity atom types are different; the    type and quantity of the second impurity atom type is selected to    reduce the detrimental effect on the colour caused by the first    impurity atom type so as to produce a diamond layer having high    colour; and the first and second impurity atom types are    independently nitrogen or atoms which are solid in the elemental    state.

In the method of this aspect of the invention, the first impurity atomtype is nitrogen and the second impurity atom type is selected fromsilicon or boron. In this way, the addition of a gaseous sourcecomprising silicon or boron impurity atom types counters the detrimentaleffect on colour of diamond that nitrogen would otherwise have.

Alternatively, the first impurity atom type is silicon or boron and thesecond impurity atom type is nitrogen. In this way, the addition of agaseous source comprising nitrogen impurity atoms counters thedetrimental effect on colour of diamond that either silicon or boronwould have.

Where the first or second impurity atom type is nitrogen, the first orsecond gas may be any gaseous species which contains nitrogen includingN₂, NH₃ (ammonia), N₂H₄ (hydrazine) and HCN (hydrogen cyanide).Preferably, the first or second gas is N₂, NH₃, or N₂H₄. Preferably, thefirst or second gas is N₂ or NH₃, preferably the first or second gas isN₂. The nitrogen present in the synthesis atmosphere is calculated asparts per million (ppm) or parts per billion (ppb) of molecular nitrogen(ie N₂) as a molecular fraction of the total gas volume. Thus 100 ppb ofnitrogen added as molecular nitrogen (N₂) is equivalent to 200 ppb ofatoms of nitrogen or 200 ppb of ammonia (NH₃).

For impurity additions other than nitrogen, the gas phase concentrationin ppm or ppb refers to the concentration in synthesis atmosphere of theimpurity added as the preferred gaseous species.

Where the first or second impurity atom type is boron, the first orsecond gas is preferably B₂H₆, BCl or BH₃. Preferably, the first orsecond gas is B₂H₆.

Where the first or second impurity atom type is silicon, the first orsecond gas is preferably SiH₄, or Si₂H₆. Preferably, the first or secondgas is SiH₄.

For silicon and boron, if gaseous species other than the preferredspecies (ie B₂H₆, SiH₄) are used to add the impurity atom type to thesynthesis environment, the number of atoms of the impurity atom type inthe molecular species added must be accounted for in determining theconcentration of that species in the synthesis environment.

The impurity atom types are added to the synthesis atmosphere as gases.Although it is possible, with the exception of nitrogen, to add all theimpurity atom types as single element solids, it is extremely difficult,if not impossible, to accurately and reproducibly control the rate atwhich such additions are made. For example, additions of boron have beenmade by exposing solid boron to the synthesis atmosphere; the sameapplies to silicon where solid sources have been used. However, gaseoussources of the impurity atom types are used in the method of the presentinvention because the gaseous source of an impurity atom type may beprepared in a highly pure form, diluted gravimetrically with a carriergas and then analysed post-manufacture to accurately determine the exactconcentration. Given the gas concentration, precise and reproducibleadditions can be added using gas metering devices such as mass-flowcontrollers.

The incorporation of an impurity atom type from the synthesis atmosphereinto the solid diamond is highly dependent upon the exact details of thesynthesis process. Such matters have been well detailed in the prior artand are well known to those skilled in the art. Parameters thatinfluence the level of incorporation include the nature of the molecularspecies used to provide the impurity atom, the temperature of thesynthesis atmosphere, the pressure of the synthesis atmosphere, thetemperature of the surface of the substrate, the crystallographic natureof the surface and the gas flow conditions with the synthesis system.

Where the first or second gas source comprises nitrogen, theconcentration of the gas comprising nitrogen in the synthesis atmospheremay be greater than 300 ppb, greater than 500 ppb, greater than 600 ppb,greater than 1 ppm, greater than 2 ppm, greater than 3 ppm, greater than5 ppm, greater than 10 ppm, greater than 20 ppm, greater than 30 ppm.The concentration of the gas comprising nitrogen may be in the rangefrom 300 ppb to 30 ppm, 500 ppb to 20 ppm, 600 ppb to 10 ppm, 1 ppm to 5ppm, or 2 ppm to 3 ppm.

When the first or second gas source comprises boron, the concentrationof the gas comprising boron in the synthesis atmosphere may be greaterthan 0.5 ppb, greater than 1.0 ppb, greater than 2 ppb, greater than 5ppb, greater than 10 ppb, greater than 20 ppb, greater than 50 ppb,greater than 0.1 ppm, greater than 0.2 ppm. The concentration of the gascomprising boron in the synthesis atmosphere may be from 0.5 ppb to 0.2ppm, from 1.0 ppb to 0.1 ppm, from 2 ppb to 50 ppb, from 10 ppb to 20ppb. The concentration of the gas comprising boron in the synthesisatmosphere may be less than 1.4 ppm, less than 0.1 ppm, or less than0.05 ppm.

When the first or second gas source comprises silicon, the concentrationof the gas comprising silicon in the synthesis atmosphere may be greaterthan 0.01 ppm, greater than 0.03 ppm, greater than 0.1 ppm, greater than0.2 ppm, greater than 0.5 ppm, greater than 1 ppm, greater than 2 ppm,greater than 5 ppm, greater than 10 ppm, greater than 20 ppm. Theconcentration of the gas source comprising silicon in the synthesisatmosphere may be from 0.01 ppm to 20 ppm, 0.03 ppm to 10 ppm, 0.1 ppmto 5 ppm, 0.2 ppm to 2 ppm, or 0.5 ppm to 1 ppm.

Secondary Ion Mass Spectrometry (SIMS) measurements have shown that, fora given concentration of silicon in the growth gases, in the absence ofnitrogen, the concentration of silicon in the grown diamond is higherfor {111}, {110} or {113} growth than for {100} growth. For growth on asubstrate with {100} orientation, although gaseous silicon impuritytends to increase the threshold nitrogen concentration for surfaceroughening, addition of high concentrations of nitrogen to the growthgases will eventually cause the surface to roughen and the efficiency ofsilicon incorporation to increase dramatically. When this happens SIMSmeasurements indicate that the concentration of silicon in the diamondcan significantly exceed that of nitrogen and in such cases the diamondwill generally show a grey colour resulting from high concentrations ofthe defect responsible for a spectroscopic feature at 945 nm in theabsorption spectrum (currently believed to be a neutral silicon-vacancydefect). In general, as the concentration of gaseous silicon isincreased the grey colour is perceived earlier for material grown on{111}, {110} or {113} than for {100} growth.

When silicon is the first or the second impurity atom type, theconcentration of silicon in the majority volume of the diamond layerproduced may be less than or equal to 2×10¹⁸ atoms/cm³. Theconcentration of silicon in the majority volume of the diamond layer maybe in the range from 10¹⁴ atoms/cm³ to 2×10¹⁸ atoms/cm³, from 3×10¹⁴atoms/cm³ to 10¹⁷ atoms/cm³, from 10¹⁵ atoms/cm³ to 3×10¹⁶ atoms/cm³, orfrom 3×10¹⁵ atoms/cm³ to 10¹⁶ atoms/cm³. The concentration of silicon inthe majority volume of the diamond layer may be greater than 10¹³atoms/cm³, greater than 10¹⁴ atoms/cm³, greater than 3×10¹⁴ atoms/cm³,greater than 10¹⁵ atoms/cm³, greater than 3×10¹⁵ atoms/cm³, greater than10¹⁶, greater than 3×10¹⁶ atoms/cm³, greater than 10¹⁷ atoms/cm³.

When nitrogen is the first or the second impurity atom type, theconcentration of nitrogen in the majority volume of the diamond layermay be from 1×10¹⁴ atoms/cm³ to 5×10¹⁷ atoms/cm³, from 5×10¹⁵ atoms/cm³to 2×10¹⁷ atoms/cm³, or from 1×10¹⁶ to 5×10¹⁶ atoms/cm³. Theconcentration of nitrogen in the majority volume of the diamond layermay be greater than 2×10¹⁵ atoms/cm³, greater than 5×10¹⁵ atoms/cm³,greater than 10¹⁶ atoms/cm³, greater than 3×10¹⁶ atoms/cm³, greater than10¹⁷ atoms/cm³.

When boron is the first or the second impurity atom type, theconcentration of boron in the majority volume of the diamond layer maybe from 10¹⁴ atoms/cm³ to 10¹⁸ atoms/cm³, from 3×10¹⁴ atoms/cm³ to 10¹⁷atoms/cm³, from 10¹⁵ atoms/cm³ to 10¹⁶ atoms/cm³, or from 3×10¹⁵atoms/cm³ to 10¹⁶ atoms/cm³. The concentration of boron in the majorityvolume of the diamond layer may be greater than 10¹³ atoms/cm³, greaterthan 10¹⁴ atoms/cm³, greater than 3×10¹⁴ atoms/cm³, greater than 10¹⁵atoms/cm³, greater than 3×10¹⁵ atoms/cm³, greater than 10¹⁶, greaterthan 3×10¹⁶ atoms/cm³, greater than 10¹⁷ atoms/cm³.

Typically, the concentration of the first and second impurity atomtypes, as well as the concentration of any other impurities in thediamond layer, may be measured using secondary ion mass spectroscopy(SIMS). Detection limits for impurity atoms vary depending on the SIMSconditions used. However, SIMS detection limits for the first and secondimpurity atom types of the present invention typically lie in the range10¹⁴ to 10¹⁷ atoms/cm³. In particular, for elements such as boron andsilicon the detection limits are typically about 10¹⁵ atoms/cm³, whereasfor nitrogen they are typically about 10¹⁶ atoms/cm³. Other techniques,such as combustion analysis, absorption, EPR, can give highersensitivity in some instances.

When the first and second impurity atom types are nitrogen and silicon,respectively or vice versa, the concentration of nitrogen in themajority volume of the diamond layer is preferably less than or equal to2×10¹⁷ atoms/cm³ and the concentration of silicon in the majority volumeof the diamond layer is preferably less than or equal to 2×10¹⁸atoms/cm³. In this way, high colour in the synthesised diamond may bemore readily achieved.

When the first and second impurity atom types are nitrogen and silicon,respectively or vice versa, the ratio of the concentration of nitrogento silicon in the majority volume of the diamond layer produced may be1:20 to 20:1, 1:10 to 10:1, 1:9 to 9:1, 1:8 to 8:1, 1:7 to 7:1, 1:6 to6:1, 1:5 to 5:1, 1:4 to 4:1, 1:3 to 3:1, 1:2 to 2:1, preferably 1:1.

When the first and second impurity atom types are nitrogen and silicon,respectively or vice versa, the gas comprising nitrogen may be presentin the synthesis atmosphere at a concentration of greater than 100 ppb,greater than 200 ppb, greater than 300 ppb and the gas comprisingsilicon may be present in the synthesis atmosphere at a concentration ofgreater than 10 ppb.

When the first and second impurity atom types are nitrogen and boron,respectively or vice versa, the ratio of the concentration of nitrogento the concentration of boron in the majority volume of the diamondlayer may be in the range from 1:2 to 2:1, from 2:3 to 3:2, from 3:4 to4:3, from 4:5 to 6:5, from 9:10 to 11:10, preferably the ratio is 1:1.Preferably, the ratio of nitrogen to boron is greater than 1:5.

When single substitutional boron and nitrogen are present in diamond inapproximately the same concentrations, colourless material can resultbecause the nitrogen defects donate electrons to the boron defects andthe resultant ionised defects do not give rise to significant opticalabsorption. Thus, not only does boron have a beneficial effect on growthin the presence of nitrogen because of the fact that it suppressesroughening of the growth surface, boron and nitrogen incorporated intothe diamond can compensate each other to give material with low opticalabsorption. By “low optical absorption” is meant that a material absorbslittle in the visible spectrum. In particular, a diamond layer has lowoptical absorption if at least 50% of the diamond layer (the “majorityvolume”) has an absorption coefficient which at all wavelengths between300 and 1000 nm is less than 20 cm−1. A diamond layer having low opticalabsorption may have an absorption coefficient at 270 nm of less than 2cm−1, and/or an absorption coefficient at 350 nm which is less than 1.5cm−1, and/or an absorption coefficient at 520 nm of less than 1 cm−1.All absorption coefficients being measured at room temperature.

When the first and second impurity atom types are nitrogen and boron,respectively or vice versa, the gas comprising nitrogen may be presentin the synthesis atmosphere at a concentration of greater than 100 ppb,preferably greater than 200 ppb, preferably greater than 300 ppb and thegas comprising boron may be present in the synthesis atmosphere at aconcentration of greater than 0.5 ppb.

Preferably, the CVD diamond layer produced by any of the above methodshas an increased normalized free exciton intensity compared to a methodwhere the second gas comprising a second impurity type atom is notadded. Preferably, there is a strong free exciton luminescence in thecathodoluminescence spectrum measured at 77 K, with the integratedintensity of the free exciton luminescence exceeding 0.3, preferablyexceeding 0.4, preferably exceeding 0.5, preferably exceeding 0.6,preferably exceeding 0.7, preferably exceeding 0.8, preferably exceeding0.9 of the integrated free exciton luminescence intensity for ahomoepitaxial CVD diamond sample grown under high purity conditions.

The CVD diamond layer produced by any of the above methods may have anincrease in carrier mobility, carrier lifetime, charge collectiondistance and/or charge collection efficiency compared to a method wherethe second gas comprising a second impurity type atom is not added. Thecharge collection distance of the produced diamond layer may be greaterthan 100 μm, greater than 150 μm, greater than 200 μm, greater than 300μm, greater than 500 μm, or greater than 1000 μm when measured with anapplied electric field of 1.0 V/μm. A method of measuring chargecollection distance in diamond is described in WO 01/96633, for example.The carrier mobility of the produced diamond layer may be 1200cm²V⁻¹s⁻¹, preferably 1500 cm²V⁻¹s⁻¹, preferably 1800 cm²V⁻¹s⁻¹,preferably 2200 cm²V⁻¹s⁻¹, preferably 2500 cm²V⁻¹s⁻¹. Preferably thecharge collection efficiency of the produced diamond layer is 30%,preferably 50%, preferably 70%, preferably 80%, preferably 90%,preferably 95%, preferably 97%. The carrier lifetime of the produceddiamond layer may be greater than 1 ns, greater than 3 ns, greater than10 ns, greater than 30 ns, or greater than 100 ns.

Nitrogen as an impurity is known to affect the electronic properties ofsingle crystal CVD diamond, in particular the charge collectiondistance, carrier mobilities and carrier lifetimes. In the absence ofnitrogen the electronic properties of single crystal CVD diamond can bevery good (see for example Isberg et al, Science, volume 297, pages1970-1672, where methods of measurement and results are disclosed). Asnitrogen is progressively added to the synthesis atmosphere, theelectronic properties of the resultant material are progressivelydegraded.

Previous experimentation has shown that the intensity of the freeexciton emission at 235 nm measured at 77 K is a good proxy for theelectronic properties (WO 01/96633). Using this proxy, we are able topropose the following expected behaviour for combined nitrogen andsilicon additions to diamond.

If silicon is added with nitrogen, the deleterious effects of thenitrogen on electronic properties are ameliorated, with the amount ofamelioration increasing, but the rate of amelioration decreasing, as theconcentration of silicon added is increased, until at some fraction ofthe concentration of nitrogen being incorporated, adding further siliconceases to have a further ameliorating effect at which point theproperties start to degrade once more.

Therefore there will be an optimum amount of silicon that can be addedfor a given amount of nitrogen in the solid, but the optimum value isdependent on the exact property that is being considered and the amountof nitrogen incorporated. The inventors expect that the optimum value ofsilicon addition with regard to its effect on the electronic propertiesis generally somewhat less than the silicon addition at which the colourof the diamond starts degrading (ie the greyness caused by siliconbegins to become apparent).

Thus, it is possible for a series of diamonds containing a givenconcentration of nitrogen and different concentrations of silicon(ranging from just above zero to well beyond the optimum) to haveelectronic properties that can be slightly better, much better, the sameor worse than an otherwise identical diamond containing no silicon.

It is equally possible for a diamond of the invention to have poorelectronic properties (i.e. the silicon concentration is well beyond theoptimum), but good optical properties as the greyness caused by thesilicon has not yet become sufficient to be perceived as a colour changeor caused a significant change to the optical absorption spectrum.

A similar situation pertains when boron is added to a synthesisatmosphere containing diamond. Initially the boron ameliorates thedeleterious effects of the nitrogen and the electronic propertiesimprove. As the amount of boron added is increased, at some point,probably when the amounts of nitrogen and boron are approximately equal,the improvement in the electronic properties will stop and then, withhigher rates of addition, begin to decline. This behaviour can beunderstood with a classical semiconductor compensation model. The rateof improvement and subsequent decline in the properties is expected tobe much sharper than for the case of nitrogen and silicon.

The first gas comprising a first impurity atom type may be deliberatelyadded to the synthesis atmosphere. Alternatively, the first gas may bepresent in the synthesis atmosphere unintentionally, including beingpresent in the synthesis atmosphere because it has not been removed eventhough it affects properties of the diamond layer produced. Preferably,the synthesis atmosphere comprises a concentration of the first gas,which has not been added deliberately, of greater than 0.1 ppb,preferably greater than 1 ppb, preferably greater than 10 ppb. Anexample of such a situation is where nitrogen remains in the synthesisatmosphere in the form of NH₃, air or N₂H₄, for example, and it isconsidered too expensive or time consuming to adopt extra measures toremove such gases from the synthesis atmosphere. Preferably, thesynthesis atmosphere comprises a concentration of the gas comprisingnitrogen, which has not been added deliberately, of greater than 300ppb.

The first gas may be present in the synthesis atmosphere in a mannerwhich is controlled or in a manner which is not controlled. Where thefirst gas is present in a manner which is not controlled, the firstimpurity type atom may be present as an impurity of one of the gasesrequired for diamond synthesis. Alternatively, where the first gas isadded in a manner which is controlled, this may be such that there isonly an upper limit of the amount of gas that may be introduced into thesynthesis atmosphere. Alternatively, the presence of the first gas maybe controlled such that the concentration of the first gas is stable tobetter than 20%, preferably better than 10%, preferably better than 3%.

Preferably, the diamond layer is greater than 0.1 mm thickness,preferably greater than 0.5 mm thickness, preferably greater than 1 mmthickness, preferably greater than 2 mm thickness.

In the method of the first embodiment:

-   -   (1) the substrate may be a diamond substrate having a surface        which is substantially free of crystal defects such that a        revealing plasma etch would reveal a density of surface etch        features related to defects below 5×10³/mm²;    -   (2) the duration of the synthesis of the diamond layer may be at        least 50 hours; and/or    -   (3) the substrate may comprises multiple separated single        crystal diamond substrates.

The method may comprise at least one, preferably at least two,preferably all three of features (1) to (3). The method may comprisefeature (1), feature (2), feature (3), features (1) and (2), features(1) and (3), features (2) and (3), or features (1), (2) and (3).

By using a diamond substrate having a surface which is substantiallyfree of crystal defects, the quality of the grown diamond can be greatlyimproved. In particular, fewer defects will be present in the growndiamond layer.

The duration of the synthesis of the diamond layer may be at least 50hours, at least 75 hours, at least 100 hours, at least 150 hours.

In a further embodiment of the present invention there is provided amethod of producing a CVD single crystal diamond layer, comprising:

-   -   (i) providing a substrate;    -   (ii) providing a CVD synthesis atmosphere in which there exists        a concentration of nitrogen which is not deliberately added of        greater than 300 ppb; and    -   (iii) adding into the synthesis atmosphere a second gas        comprising a second impurity atom type other than nitrogen,

wherein the second impurity atom type is added in a controlled manner inan amount that reduces the detrimental effect on the colour caused bythe nitrogen so as to produce a diamond layer having high colour; andthe second impurity atom type is solid in the elemental state.

In this way, a CVD diamond layer having high colour may be produced eventhough the synthesis atmosphere comprises an amount of nitrogen that, inthe absence of the second gas, would have an undesirable affect on thecolour of the produced diamond such that the produced diamond would nothave high colour. The method of the second embodiment enables highcolour CVD diamond to be synthesised without having to take anyadditional steps to remove the undesirable nitrogen from the synthesisatmosphere. The term “high colour” is as defined previously. Preferably,the diamond layer has very high colour, as defined previously.

In a still further embodiment of the present invention there is provideda method of producing a CVD single crystal diamond layer comprising thesteps of:

-   -   (i) providing a substrate; and    -   (ii) adding into a CVD synthesis atmosphere a gaseous source        comprising silicon.

In this way a silicon doped diamond layer is provided.

In the method of this embodiment of the invention:

-   -   (1) the layer may be grown to greater than 0.1 mm thickness;    -   (2) the substrate may be a diamond substrate having a surface        which is substantially free of crystal defects such that a        revealing plasma etch would reveal a density of surface etch        features related to defects below 5×10³/mm²;    -   (3) the duration of the synthesis of the single crystal diamond        layer may be at least 50 hours; and/or    -   (4) the substrate may comprise multiple separated single crystal        diamond substrates.

The method may comprise at least one, at least two, at least three,preferably all four of features (1) to (4). The method may comprisefeature (1), feature (2), feature (3), feature (4), features (1) and(2), features (1) and (3), features (1) and (4), features (2) and (3),features (2) and (4), features (3) and (4), features (1), (2) and (3),features (1), (3) and (4), features (2), (3) and (4),

The layer may be grown to a thickness of greater than 0.5 mm, greaterthan 1 mm, greater than 2 mm.

The duration of the synthesis of the diamond layer may be at least 50hours, at least 75 hours, at least 100 hours, at least 150 hours.

The preferred features of the first embodiment of the method of thepresent invention outlined above apply equally to the third embodimentof the method of the present invention as long as the first or secondsource gases comprise silicon.

The concentration of silicon in the majority volume of the diamond layerproduced by the third embodiment of the method of the present inventionmay be up to 2×10¹⁸ atoms/cm³, from 10¹⁴ atoms/cm³ to 2×10¹⁸ atoms/cm³,from 3×10¹⁴ atoms/cm³ to 10¹⁷ atoms/cm³, from 10¹⁵ atoms/cm³ to 3×10¹⁶atoms/cm³, from 3×10¹⁵ atoms/cm³ to 10¹⁶ atoms/cm³, from 2×10¹⁷ to2×10¹⁸ atoms/cm³.

In the third embodiment of the method of the present invention, theaddition of silicon may reduce an adverse effect on a property of theproduced diamond layer caused by the presence of an impurity atom type.Preferably, the impurity atom type is nitrogen. The impurity atom typemay be introduced into the synthesis atmosphere as a gas in a controlledor uncontrolled manner, as described previously. Preferably, theimpurity atom type is nitrogen and the synthesis atmosphere comprises aconcentration of nitrogen which is not deliberately added of greaterthan 300 ppb.

The property may be colour and adding silicon may produce a CVD diamondlayer having high colour, wherein “high colour” is as defined above.Preferably, the CVD diamond layer has very high colour, wherein “veryhigh colour” is as defined above.

The property may be free exciton emission of the diamond layer andadding silicon may produce a CVD diamond layer with an increasednormalized free exciton intensity compared to a method where silicon isnot added. There may be a strong free exciton luminescence in thecathodoluminescence spectrum measured at 77 K, with the integratedintensity of the free exciton luminescence exceeding 0.3, preferablyexceeding 0.4, preferably exceeding 0.5, preferably exceeding 0.6,preferably exceeding 0.7, preferably exceeding 0.8, preferably exceeding0.9 of the integrated free exciton luminescence intensity for ahomoepitaxial CVD diamond sample grown under high purity conditions.

The property may be at least one of: carrier mobility; carrier lifetime;and charge collection distance and adding silicon may produce a CVDdiamond layer with an increase in carrier mobility, carrier lifetimeand/or charge collection distance compared to a method where silicon isnot added. The charge collection distance of the produced diamond layermay be greater than 100 μm, greater than 150 μm, greater than 200 μm,greater than 300 μm, greater than 500 μm, greater than 1000 μm whenmeasured with an applied electric field of 1.0 V/μm. The carriermobility of the produced diamond layer may be 1200 cm²V⁻¹s⁻¹, preferably1500 cm²V⁻¹s⁻¹, preferably 1800 cm²V⁻¹s⁻¹, preferably 2200 cm²V⁻¹s⁻¹,preferably 2500 cm²V⁻¹s⁻¹. The charge collection efficiency of theproduced diamond layer may be 30%, preferably 50%, preferably 70%,preferably 80%, preferably 90%, preferably 95%, preferably 97%. Thecarrier lifetime of the produced diamond layer may be greater than 1 ns,greater than 3 ns, greater than 10 ns, greater than 30 ns, greater than100 ns.

In any of the methods described above (that is, for the first, secondand third embodiments), when the CVD diamond layer is a single crystal,the majority volume of the diamond layer may have at least one of thefollowing features:

-   -   a) an absorption spectrum measured at room temperature such that        the colour of a standard 0.5 ct round brilliant would be better        than K;    -   b) an absorption coefficient at 270 nm measured at room        temperature which is less than 1.9 cm⁻¹;    -   c) an absorption coefficient at 350 nm measured at room        temperature which is less than 0.90 cm⁻¹;    -   d) an absorption at 520 nm of less than 0.30 cm⁻¹; or    -   e) an absorption at 700 nm of less than 0.12 cm⁻¹.

The majority volume of the diamond layer may comprise at least 55%, atleast 60%, preferably at least 70%, preferably at least 80%, preferablyat least 90%, preferably at least 95% of the diamond layer.

The single crystal diamond layer may have at least two, at least three,at least four, preferably all five of the features (a) to (e). Thediamond layer may have features a) and b); features a) and c); a) andd); a) and e); b) and c); b) and d); b) and e); c) and d); c) and e); d)and e); a), b) and c); a), b) and d); a), b) and e); a), c) and d); a),c) and e); a), d) and e); b), c) and d); b), c) and e); b), d) and e);c), d) and e); a), b), c) and d); a), b), c) and e); a), b), d) and e);a), c), d) and e); b), c), d) and e); or a), b), c), d) and e).

Preferably, for feature a), the diamond layer has an absorption spectrummeasured at room temperature such that the colour of a standard 0.5 ctround brilliant would be better than J, preferably better than I;preferably better than H, preferably better than G, preferably betterthan F, preferably better than E, preferably D.

Preferably, for feature b), the diamond layer has an absorptioncoefficient at 270 nm measured at room temperature which is less than1.0 cm⁻¹; preferably less than 0.4 cm⁻¹.

Preferably, for feature c), the diamond layer has an absorptioncoefficient at 350 nm measured at room temperature which is less than0.5 cm⁻¹; preferably less than 0.2 cm⁻¹.

Preferably, for feature d), the diamond layer has an absorptioncoefficient at 520 nm measured at room temperature which is less than0.14 cm⁻¹; preferably less than 0.06 cm⁻¹.

Preferably, for feature e), the diamond layer has an absorptioncoefficient at 700 nm measured at room temperature which is less than0.06 cm⁻¹; preferably less than 0.03 cm⁻¹.

In any of the methods outlined above, the diamond layer may be formedinto a gemstone having three orthogonal dimensions greater than 2 mm,where at least one axis lies either along the <100> crystal direction oralong the principle symmetry axis of the gemstone. The inventiondescribed above further provides a CVD diamond produced from a singlecrystal CVD layer described above polished in the form of a gemstonecharacterised by having three orthogonal dimensions greater than 2 mm,and preferably greater than 2.5 mm, and more preferably greater than 3.0mm, where at least one axis lies either along the <100> crystaldirection or along the principle symmetry axis of the stone. The diamondwill be of high quality and may have one or more of the characteristicsidentified above.

According to the present invention there is provided a CVD singlecrystal diamond layer produced by any one of the methods disclosedabove. The majority volume of the diamond layer may be formed from asingle growth sector.

In view of the reduction in defects in the diamond layer produced by anyof the methods of the invention described above because of a reductionin surface roughening during growth, the diamond layer may also haveimproved mechanical and chemical properties, including wear resistanceand thermal stability. The wear properties of a material are the resultof very complex interactions between a wide range of the macroscopicproperties of the material including, for example, its hardness,strength, stiffness, toughness, grain size, thermal conductivity, grainorientation etc. It is well known in the art that diamond hasexceptional wear properties and these are widely exploited: it is usedas a tool material in a wide range of applications including cuttingtools, rock drill, wire dies and many others.

The performance of a diamond tool in a particular application isstrongly influenced by its microstructure and extended defect densities.A particular example is a wire drawing die as disclosed inWO2004/074557, where reducing the strain by controlling extended defectdensity is shown to be particularly effect at improving the wearproperties. Since the methods of the invention can provide singlecrystal diamond material with reduced point and extended defectdensities compared with diamond prepared using generally the same methodwithout the added second impurity, it can be reasonably expected thatthe material of the invention will have improved wear properties.

According to the present invention there is also provided a CVD singlecrystal diamond layer comprising an impurity atom type which is silicon,wherein the diamond layer has high colour. The concentration of thesilicon in the majority volume of the diamond layer is preferably from10¹⁴ to 2×10¹⁸ atoms/cm³. For example, the concentration of silicon inthe majority volume of the diamond layer may be greater than 10¹³atoms/cm³, greater than 10¹⁴ atoms/cm³, greater than 3×10¹⁴ atoms/cm³,greater than 10¹⁵ atoms/cm³, greater than 3×10¹⁵ atoms/cm³, greater than10¹⁶, greater than 3×10¹⁶ atoms/cm³, greater than 10¹⁷ atoms/cm³.Preferably, the majority volume of the CVD single crystal diamond layercomprises from 2×10¹⁷ to 2×10¹⁸ atoms/cm³ of silicon.

Photoluminescence spectroscopy offers a sensitive method for detectingthe presence of silicon-related defects in diamond. A silicon-relatedphotoluminescence line at 737 nm can generally be detected with 633 nmHeNe laser excitation at 77 K. Research by the present inventors hasindicated that the photoluminescence spectrum of silicon-doped diamond,excited at 77 K with 785 nm laser radiation, also often shows a line at946 nm. This is generally accompanied by another line at 975 nm. Thesetwo photoluminescence lines have not been reported before. FIG. 21 showsa typical photoluminescence spectrum for silicon-doped diamond excitedwith 785 nm laser radiation.

The present inventors have also investigated silicon-doped samples usingEPR (Electron Paramagnetic Resonance). This offers a sensitive methodfor detecting and characterising silicon-related defects. The currentdetection limits allow defect concentrations as low as one part perbillion to be measured. A neutral silicon-vacancy defect has recentlybeen detected and characterised using EPR, and work is proceeding toidentify other silicon related defects in the same way. Current resultssuggest that the 946 nm photoluminescence line may be an opticalsignature of the neutral silicon vacancy defect identified using EPR.

Preferably, the CVD diamond layer has high colour, wherein “high colour”is as defined above.

The CVD diamond layer produced by any of the methods of the presentinvention may have a birefringence of less than 1×10⁻³, preferably lessthan 1×10⁻⁴, preferably less than 3×10⁻⁴, preferably less than 1×10⁻⁵over a volume greater than 0.1 mm³, preferably greater than 0.5 mm³,preferably greater than 1 mm³, preferably greater than 3.4 mm³,preferably greater than 8 mm³, preferably greater than 27 mm³,preferably greater than 64 mm³, preferably greater than 125 mm³,preferably greater than 512 mm³, preferably greater than 1000 mm³.Birefringence may be characterized using, for example, Metripol®apparatus.

For an isotropic medium, such as stress-free diamond, the refractiveindex is independent of the direction of the polarization of light. If adiamond sample is inhomogeneously stressed, either because of grown-instress or local defects or because of externally applied pressure, therefractive index is anisotropic. The variation of the refractive indexwith direction of polarization may be represented by a surface calledthe optical indicatrix that has the general form of an ellipsoid. Thedifference between any two ellipsoid axes is the linear birefringencefor light directed along the third.

This may be expressed as a function involving the refractive index ofthe unstressed material, the stress and opto-elastic coefficients.

Metripol® (Oxford Cryosystems) gives information on how the refractiveindex at a given wavelength depends on polarization direction in theplane perpendicular to the viewing direction. An explanation of how theMetripol® works is given by A. M. Glazer et al. in Proc. R. Soc. Lond. A(1996) 452, 2751-2765.

The Metripol® instrument determines the direction of the “slow axis”,i.e. the polarization direction in the plane perpendicular to theviewing direction for which the refractive index is a maximum. It alsomeasures |sin δ| where δ is the phase shift given byδ=(2π/λ)Δn Lwhere λ is the wavelength of the light, L is the thickness of thespecimen and Δn is the difference between the refractive index for lightpolarized parallel to the slow and fast axes i.e. the birefringence. ΔnL is known as the ‘optical retardation’.

For retardation in first order, with L=0.6 mm and λ=589.6 nm, then: whensin δ=1 and Δn L=λ/4, it can be deduced that Δn=2.45×10⁻⁴. when sinδ=0.5 and Δn L=λ/12, it can be deduced that Δn=0.819×10⁻⁴.

Metripol® produces three colour-coded images showing the spatialvariations of a) the “slow axis”, b) |sin δ| and c) the absorbance atthe wavelength of operation.

Samples are prepared as optical plates of known thickness and analysedover an area of at least 1.3 mm×1.3 mm, and preferably 2.5 mm×2.5 mm,and more preferably 4 mm×4 mm. Metripol® |sin δ| images are thenanalysed and the maximum value of |sin δ| in each frame over the wholeof the analysis area and use these values to characterise the maximumvalue of Δn can be calculated of the whole of the area analysed.

The behaviour of sine δ is the property of a particular plate ofmaterial, constrained here to plates of useful thickness by applicationof a minimum thickness. A more fundamental property of the material canbe obtained by converting the sine δ information back to a valueaveraged over the thickness of the sample of the difference between therefractive index for light polarised parallel to the slow and fast axes,Δn_([average]).

Instrument resolution and noise sets a lower limit to the value of |sinδ| and hence the retardation Delta-n.d measurable by Metripol®. This inturn sets a lower limit on the measurable birefringence, although thelimit on this parameter depends on the specimen thickness. Forillustration, if the lower limit on |sin delta| is 0.03, for light ofwavelength 550 nm, this corresponds to a lower limit on the measurablebirefringence of Δn=1.05×10⁻⁵ for a sample of thickness 500 microns; ora lower limit on the measurable birefringence of Δn=7.5×10⁻⁷ for asample of thickness 3500 microns.

Birefringence values may be determined in 3 orthogonal directions whicheffectively enable a volume measurement. This may be particularlyimportant in some applications such as spherical optics etc. The limitsdefined below are calculated based on measurements and assuming a 3 mmpath length.

Preferably, the methods of this invention provide for the fabrication ofdiamond material such that birefringence measurements in at least one,preferably two, preferably all three orthogonal directions show valuesof Δn such that:

preferably Δn is less than 2×10⁻⁶ over areas greater than 1×1 mm,preferably over areas greater than 2×2 mm, preferably over areas greaterthan 4×4 mm, preferably over areas greater than 7×7 mm, preferably overareas greater than 15×15 mm;

preferably Δn is less than 5×10⁻⁶ over areas greater than 1×1 mm,preferably over areas greater than 2×2 mm, preferably over areas greaterthan 4×4 mm, preferably over areas greater than 7×7 mm, preferably overareas greater than 15×15 mm;

preferably Δn is less than 1×10⁻⁵ over areas greater than 1×1 mm,preferably greater than 2×2 mm, preferably greater than 4×4 mm,preferably greater than 7×7 mm, preferably greater than 15×15 mm.

Where birefringence values lie below a given threshold for each of threeorthogonal directions of a particular volume of diamond, then for thepurposes of this specification that volume is deemed to have abirefringence value below that threshold.

The present invention also provides a CVD diamond layer producedaccording to any of the methods outlined above for use as an opticalelement.

The present invention also provides a CVD diamond layer producedaccording to any of the methods outlined above for use as an electricalor electronic element. The present invention also provides a CVD diamondlayer produced according to any of the methods above for use as acutting tool or wire drawing die or other wear-resistant part.

The present invention also provides a CVD single crystal diamond layerproduced according to any of the methods outlined above wherein thediamond layer is in the form of a gemstone.

Preferably, the CVD single crystal diamond has three orthogonaldimensions greater than 2 mm, wherein at least one axis lies along the<100> crystal direction or along the principle symmetry axis of thegemstone. Preferably, the three orthogonal dimensions are greater than2.5 mm, preferably greater than 3.0 mm, preferably greater than 3.5 mm.Preferably, the CVD single crystal diamond layer is of high clarity,with clarity of at least SI1 on the GIA gem grading scale, as definedabove. Preferably, the CVD single crystal diamond layer has clarity ofat least VS2, preferably at least VVS2, preferably at least VVS1 on theGIA gem grading scale.

The diamond layer produced by any of the methods of the presentinvention is preferably of high crystalline quality. In relation tosingle crystal diamond “high crystalline quality” allows the presence ofthe impurity atoms and associated point defects but places limits on thepresence of dislocation bundles or other extended defects which impacton the use of the material for optical applications, for example bycausing excessive scattering, or colour, or reduction in strength orprocessability below that required for the intended optical application.In relation to polycrystalline diamond, “high crystalline quality” meansthat the material has a negligible content of non-diamond carbon andother defects in the grain boundaries. Such defects have a significantimpact on the usability of the material for optical and otherapplications and are, therefore, undesirable.

The present invention also provides a use of a sufficient quantity of agaseous source comprising a second impurity atom type to counter thedetrimental effect on colour of diamond of a first impurity atom type ina method of producing a CVD diamond layer having high colour, whereinhigh colour is as defined above. The preferred features relating to thegaseous sources and the first and second impurity atom types defined inrelation to the first, second and third embodiments of the method of thepresent invention apply equally to this use.

The present invention also provides a use of a gaseous source of siliconfor addition to a reaction chamber comprising a substrate and a diamondsynthesis atmosphere such that the silicon counters the detrimentaleffect of a first impurity atom type in a method of CVD diamondproduction. The preferred features relating to the gaseous sources,silicon and the first impurity atom type defined in relation to thesecond embodiment of the method of the present invention apply equallyto this use.

In all of the methods, diamond layers and uses provided by the presentinvention, there may exist additional impurities in the diamond layerproduced. Preferably, the total concentration of any additionalimpurities (not including hydrogen) is less than 5 ppm, preferably lessthan 2 ppm, preferably less than 1 ppm, preferably less than 0.5 ppm,preferably less than 0.2 ppm. The concentration of any single additionalimpurity (not including hydrogen) in the diamond layer is 2 ppm or less,preferably 1 ppm or less, preferably 0.5 ppm or less, preferably 0.2 ppmor less, preferably 0.1 ppm or less.

The single crystal CVD diamond of the invention is suitable for use inoptical applications such as diamond windows, diamond lenses and anvils,and for shaping into a gemstone, and in particular a gemstone of highcolour grade.

Applications to which the CVD diamond produced by the methods of thepresent invention may be put include optical applications, such as infrared transmission windows, and etalons where control of stress andminimisation of birefringence are important, knife blades, electroniccomponents, such as Schottky diodes, and radiation detectors.

The methods of the invention described above provide for production ofhigh colour, low optical absorption single crystal CVD diamond that issuitable for optical and gem applications.

A particular example is the addition of silicon to a CVD diamondsynthesis atmosphere containing nitrogen, which has been shown toincrease the normalised FE intensity, and is thus anticipated tosubstantially improve other electronic properties toward the valueswhich have been measured in the absence of nitrogen. Additionally,reduction in defects in the diamond due to reduced surface rougheningduring growth is anticipated to improve a number of other mechanical andchemical properties, including wear resistance and thermal stability. Itis expected that thermal stability is particularly improved underconditions without stabilizing pressure, as might occur duringannealing.

It is important for the production of the uniformly high colour singlecrystal CVD diamond material of this invention described above thatgrowth takes place on a diamond surface which is substantially free ofcrystal defects. In this context, defects primarily mean dislocationsand micro cracks, but also include twin boundaries, point defects, lowangle boundaries and any other disruption to the crystal structure.Preferably the substrate is a low birefringence type Ia natural, Ib orIIa high pressure/high temperature synthetic diamond or a CVDsynthesised single crystal diamond. Defects can degrade the material intwo ways, generating stress, cracking and associated preferred sites forcolour defect formation, and adversely affecting the local uptake ofimpurities. Since dislocation multiplication occurs during the growth ofthick layers, the control of dislocations within the substrate and earlystages of growth is particularly important.

The defect density is most easily characterised by optical evaluationafter using a plasma or chemical etch optimised to reveal the defects(referred to as a revealing plasma etch), using for example a briefplasma etch of the type described below. Two types of defects can berevealed:

-   -   1) Those intrinsic to the substrate material quality. In        selected natural diamond the density of these defects can be as        low as 50/mm² with more typical values being 10²/mm², whilst in        others it can be 10⁶/mm² or greater.    -   2) Those resulting from polishing, including dislocation        structures and microcracks forming chatter tracks (sometimes        known as clatter tracks) along polishing lines. The density of        these can vary considerably over a sample, with typical values        ranging from about 10²/mm², up to more than 10⁴/mm² in poorly        polished regions or samples.

The preferred low density of defects is such that the density of surfaceetch features related to defects, as described above, is below5×10³/mm², and more preferably below 10²/mm².

The defect level at and below the substrate surface on which the CVDgrowth takes place may thus be minimised by careful preparation of thesubstrate. Included here under preparation is any process applied to thematerial from mine recovery (in the case of natural diamond) orsynthesis (in the case of synthetic material) as each stage caninfluence the defect density within the material at the plane which willultimately form the substrate surface when preparation as a substrate iscomplete. Particular processing steps may include conventional diamondprocesses such as mechanical sawing, lapping and polishing (in thisapplication specifically optimised to yield low defect levels), and lessconventional techniques such as laser processing or ion implantation andlift off techniques, chemical/mechanical polishing, and both liquid andplasma chemical processing techniques. In addition, the surface R_(Q)(root mean square deviation of surface profile from flat measured bystylus profilometer, preferably measured over 0.08 mm length) should beminimised, typical values prior to any plasma etch being no more than afew nanometers, i.e. less than 10 nanometers.

One specific method of minimising the surface damage of the substrate,is to include an in situ plasma etch on the surface on which thehomoepitaxial diamond growth is to occur. In principle this etch neednot be in situ, nor immediately prior to the growth process, but thegreatest benefit is achieved if it is in situ, because this avoids anyrisk of further physical damage or chemical contamination. An in situetch is also generally most convenient when the growth process is alsoplasma based. The plasma etch can use similar conditions to thedeposition or diamond growing process, but with the absence of anycarbon containing source gas and generally at a slightly lowertemperature to give better control of the etch rate. For example, it canconsist of one or more of:

-   -   (i) an oxygen etch using predominantly hydrogen with optionally        a small amount of Ar and a required small amount of O₂. Typical        oxygen etch conditions are pressures of 50-450×10² Pa, an        etching gas containing an oxygen content of 1 to 4 percent, an        argon content of 0 to 30 percent and the balance hydrogen, all        percentages being by volume, with a substrate temperature        600-1100° C. (more typically 800° C.) and a typical duration of        3-60 minutes.    -   (ii) a hydrogen etch which is similar to (i) but where the        oxygen is absent.    -   (iii) alternative methods for the etch not solely based on        argon, hydrogen and oxygen may be used, for example, those        utilising halogens, other inert gases or nitrogen.

Typically the etch consists of an oxygen etch followed by a hydrogenetch and then moves directly into synthesis by the introduction of thecarbon source gas. The etch time/temperature is selected to enableremaining surface damage from processing to be removed, and for anysurface contaminants to be removed, but without forming a highlyroughened surface and without etching extensively along extended defectssuch as dislocations which intersect the surface and thus cause deeppits. As the etch is aggressive, it is particularly important for thisstage that the chamber design and material selection for its componentsbe such that no material is transferred by the plasma into the gas phaseor to the substrate surface. The hydrogen etch following the oxygen etchis less specific to crystal defects, rounding off the angularitiescaused by the oxygen etch which aggressively attacks such defects andproviding a smoother, better surface for subsequent growth.

The surface or surfaces of the diamond substrate on which the CVDdiamond growth occurs are preferably the {100}, {110}, {113} or {111}surfaces. Due to processing constraints, the actual sample surfaceorientation can differ from these ideal orientations up to 5°, and insome cases up to 10°, although this is less desirable as it adverselyaffects reproducibility.

The methods of this invention described above can be further combinedwith post growth treatment, such as annealing. In this context annealingcan occur over a range of temperatures and pressures, from nearatmospheric annealing at temperatures as low as 1000° C.-1800° C., andhigh pressure annealing in the graphite or diamond stable regions attemperatures in the range 1200° C.-3000° C.

The invention will now be described with reference to the followingnon-limiting examples. In each of these examples, except whereexplicitly stated otherwise, in order to control nitrogen and thuscharacterise the utility of the invention, nitrogen was removed from theincoming gas stream by use of purifiers and high purity gas sources,such that without deliberate addition of a nitrogen dopant source thegas stream contained less than 100 ppb N₂. Nitrogen was then added backinto the process using typically a mixture of 100 ppm N₂ in hydrogen,this gas mixture giving good control of nitrogen levels in the processgases, particularly in the range of 0.5-20 ppm. Those skilled in the artwill appreciate that use of lower purity gases or poorer vacuum practicecan easily result in nitrogen impurity levels in the process,particularly in the range 1-20 ppm or greater, and in such cases thenitrogen would not be a deliberately added impurity, but one present dueto poorer control or cost savings.

The invention will now be discussed with reference to the followingFigures:

FIG. 1

A graph covering the visible wavelengths in the range 400 to 800 nm andcontaining three spectra: a) a curve centred about 450 nm (labeled450F×XEF) is the excitation beam generated by the xenon flash lamp afterfiltering by an Andover 450 nm filter, b) the rising edge of the bandpass region of the OG 550 viewing filter at about 550 nm (labeledOG550), used to remove any of the excitation frequencies from the viewedimage, and c) the emission spectrum of the 575 nm PL centre as viewedthrough the OG550 filter (labeled OG550×575) peaking in the region of620 nm

FIG. 2

A graph covering the range 200 to 800 nm showing two spectra, a) thecurve centred about 228 nm which is the excitation beam generated by thexenon flash lamp after filtering by an Andover 228 nm filter, and b) theemission spectrum of the blue PL/phosphorescence centre which peaks near500 nm and extends from 400 nm to 700 nm.

FIG. 3

A graph over the range of 200 nm to 700 nm showing the excitationspectra for a) the 575 nm band and b) the 637 nm band, up to theintrinsic diamond band edge (defining the diamond band gap). This dataand is taken from Zaitsev A., Optical Properties of Diamond: a datahandbook, Springer, 2001 (ISBN 354066582X)

FIG. 4

A graph over the range of 200 nm to 500 nm showing three spectra: a) thecurve extending across the whole wavelength range is the emissionspectrum of a Hamamatsu xenon flash lamp. The radiation from the xenonflash lamp is dominated by intense visible emission bands in the longwave range 400 nm to 550 nm and very intense short wave ultravioletemission bands in the range 220 nm to 270 nm, b) the transmission curvefor a narrow bandpass filter supplied by LOT Oriel, UK and manufacturedby Andover Corporation, USA, type 228 FS 25-25, with a peak wavelengthcentred at 228 nm, full bandwidth at half maximum of about 25 nm anddiameter of 25 mm, and c) the transmission curve for a narrow bandtransmission filter supplied by LOT Oriel, UK and manufactured byAndover Corporation, USA, type 450 FS 40-25, with a peak wavelengthcentred at 450 nm, full bandwidth at half maximum of about 40 nm anddiameter of 25 mm.

FIG. 5

A schematic representation of a suitable viewer for observing the 575 nmorange fluorescence (upper image) and blue (blue-green) phosphorescence(lower image) of a tagged synthetic cut as a gemstone. The upper image(labeled ‘575 nm orange fluor.’) shows the viewer set to excite and view575 nm PL, and relates to the spectra shown in FIG. 1. The source is thexenon flash lamp. Filter F1 is the 450 nm excitation filter and filterF3 is the orange viewing filter OG550 and relates to the spectrum shownin FIG. 1. Filter F4 could be an additional filter to reduce effectsfrom scattered light or the xenon excitation. The lower image (labeled‘blue-green phos.’) shows the viewer set to excite and view bluePL/phosphorescence, and relates to the spectrum shown in FIG. 2. Thesource is the xenon flash lamp. Filter F2 is the 228 nm excitationfilter. To prevent harmful ultraviolet light reaching the operator aglass or Perspex filter could be placed in the position marked ‘open’,in addition to the glass viewing window typically placed at the top ofthe instrument.

FIG. 6

A schematic side elevation of the viewer shown in FIG. 5. The viewingfilters are placed at approximately 45 degrees from the vertical and setaway from direct excitation to prevent the operator viewing the sourcedirectly and to eliminate the production of luminescence in the filtersfrom the excitation source.

FIG. 7

A graph over the range of 200 nm to 600 nm showing three spectra: a) the254 nm spectrum of a mercury discharge after transmission through a UG5filter, b) the 365 nm spectrum of a mercury lamp after transmissionthrough a UG5 filter, with a full width at half maximum of around 25 nm,and c) the output spectrum of a typical commercial 400 nm LED.

FIG. 8

A graph over the range of 300 nm to 800 nm showing three spectra: a) the575 nm PL band transmitted through an OG550 filter, extending fromapproximately 550-800 nm, b) the 400 nm LED excitation band with thetransmission curve of the BG25 filter, effectively extending from310-520 nm, and c) the transmission of the OG550, extending from 550 tobeyond 800 nm.

FIG. 9 a

A schematic representation of the image provided by a layer near theculet of a round brilliant. The box in the lower left is an apparatusconsisting of a ring illuminator (labeled source) positioned around aviewing window with an OG550 filter, and at the bottom of the box ispositioned a CVD synthetic cut as a round brilliant with a layer taggedwith the 575 nm PL centre occupying the volume from the culet toapproximately ⅓ the way up the pavilion facets. The illumination isbeing used to solely excite 575 nm PL. The diagram above and to theright shows the image observed in such a CVD synthetic, modeled here tobe 6 mm high with a 575 nm containing region/layer extending 0.8 mm (13%the height of the stone) from the culet, when viewed normal to thetable, as obtained by ray tracing. The eye (3 mm diameter pupil) of theobserver is about 100 mm from the culet. The ray tracing diagram wasproduced by generating 4 million rays from within the 575 nm containinglayer and calculating their trajectory within the CVD synthetic andtheir exit points. Only those rays (around 800) that entered the 3 mmaperture have been plotted on the plan view of the stone. An intense(orange, 575 nm) spot is seen in high contrast clearly evident in thecentre of the table, as well as a series of high intensity spots in thecrown facets.

FIG. 9 b

A schematic representation of the image provided by a layer near thegirdle of a round brilliant. The box in the lower left is an apparatusconsisting of a ring illuminator (labeled source) positioned around aviewing window with an OG550 filter, and at the bottom of the box ispositioned a CVD synthetic cut as a round brilliant with a layer taggedwith the 575 nm PL centre occupying a thin layer near the girdle. Theillumination is being used to solely excite 575 nm PL. The diagram aboveand to the right shows the image observed in such a CVD synthetic,modeled here to be 6 mm high with a 575 nm containing region/layerextending 0.8 mm (13% the height of the stone) from the girdledownwards, when viewed normal to the table, as obtained by ray tracing.The details for the ray tracing are as for FIG. 9 a. No intense spot isseen in the table, with some general intensity visible predominantly inthe table facets.

FIG. 10

A schematic showing a preferred apparatus to enable an observer to seethe orange/blue flash and the optical effect from an orange or blueluminescent layer in a tagged CVD synthetic.

The apparatus provides two functions. The first function comprises along wave source that only excites the orange 575 nm PL (102, 104) andwhich may be an LED (102) combined with lenses and a suitable filtersuch as BG25 (104). The 575 nm orange PL band excited in the CVD stoneis observed with high selectivity through an OG550 filter (120).

The second function comprises a short wavelength source (112, 114) thatexcites the phosphorescence efficiently, and which is preferably a xenonflash lamp (112) combined with suitable lenses and filters (114) toprovide a source of wavelengths preferably in the range 227 nm to 254nm. The glass window (110) is provided to protect the viewer from UVradiation. In this mode of operation, the OG550 (120) is removed fromthe viewing path.

The complete apparatus is contained within a darkened box to allow theluminescence to be observed without interference from stray light. Aviewing lens within the apparatus (118) allows the observer to see amagnified image of the stone.

FIG. 11

A PL (photoluminescence) image of a 0.2 carat round brilliant CVDsynthetic 10 (crown angle approximately 35 degrees to the plane of thegirdle and pavilion angle 41.5 degrees to the plane of the girdle)recorded by the DiamondView™ instrument. In the original image beforeconversion to BNV (black and white) the areas in white are showing bluePL, and the areas in black are dark. The CVD synthetic 10 has a blue PLlayer extending from the culet point to approximately 30% of the heightof the stone. The image was recorded with the table facet 12 facing theviewer. The image is dominated by a distinct blue (seen here as white)‘fish-eye’ spot 14 in the centre of the table facet 12. The remainder ofthe table facet 12 is devoid of blue PL. The crown facets 16 show adistribution of intensity from the internally scattered blue PL.

FIG. 12

On the left, a computer generated image based on ray tracing techniquesof the expected PL image for the round brilliant imaged in FIG. 11. Theside view of the modeled round brilliant (20) is shown on the right,with a PL layer 22 extending approximately 30% the height of the stone20 from the culet point 24. As in FIG. 11, the image of the PL viewedthrough the table facet is characterised by a ‘fish-eye’ spot 26 in thecentre of the table facet 28, the remainder of the table facet 28 beinglargely devoid of PL. The crown facets 30 show a distribution ofintensity from the internally scattered PL.

FIG. 13

On the left, a computer generated image based on ray tracing techniquesof the expected PL image for the round brilliant (40) whose side view isshown on the right. As in FIG. 12, this stone is modelled with a PLlayer 42 extending approximately 30% the height of the stone 40 from theculet point 44. The image of the PL viewed through the table facet ischaracterised by a ‘fish-eye’ spot 46 in the centre of the table facet28, the remainder of the table facet 28 being largely devoid of PL. Thecrown facets 30 show a weak distribution of intensity from theinternally scattered PL.

FIG. 14

A schematic diagram of a simple layer structure which could be producedin a CVD diamond layer to provide a characteristic mark using a singletype of layer with PL characteristics distinct from the bulk. Thediamond material 50 includes a pair of marker layers 52,54 which showcharacteristic PL separated by a spacer layer 56 free of this PLcharacteristic and with the characteristics of the rest of the layer.The layers 52, 54 and 56 have respective thicknesses tm1, tm2 and ts1.

FIG. 15

A schematic diagram of a simple layer structure which could be producedin a CVD diamond layer to provide a characteristic mark using two typesof layer with PL characteristics distinct from the bulk. The diamondmaterial 60 includes a pair of marker layers 62,64 which show similarcharacteristic PL separated by a layer 66 which shows PL characteristicsdistinct from layers 62 and 64, where all three layers 62, 64, 66 showPL characteristics distinct from those of the bulk of the material 60.The layers 62, 64, 66 have respective thicknesses tm1, tm2 and tn1.

FIG. 16

A schematic diagram of a more complex layer structure which could beproduced in a CVD diamond layer to provide a characteristic mark usingtwo types of layer with PL characteristics distinct from the bulk. Inparticular, layers 72, 76 and 80 and layers 74, 78 and 82 form twodistinct groups, each group of layers having PL characteristics whichare similar to each other within the group but distinct from the othergroup and from the bulk material 70. In addition the thicknesses of theindividual layers are varied.

FIG. 17

Images taken of a round brilliant cut CVD synthetic diamond using apreferred variant of the Tagging Viewer. The stone is 1.02 ct, E/Fcolour and VVS2 or better, the bulk of which shows uniform 575 nm PLexcept for a layer showing blue phosphorescence about ⅓ the way up thepavilion facets from the culet. Each image is taken looking normal tothe table facet. On the left is shown the image of 575 nm PL, where theintensity in the original image before conversion to BAN (black andwhite) was orange and now shows as lightness or white on black. One theright is shown the image of blue phosphorescence, where the intensity inthe original image before conversion to BNV was blue and now shows aslightness or white on black. The intensity in the left hand 575 nm PLimage is generally relatively uniform except for a dark ring seen in thecentre of the table facet. This corresponds to the bright ring seen inthe table in the right hand image of blue phosphorescence. Segments ofthis bright ring can also be seen in the table facets

FIG. 18

Images taken of a round brilliant cut CVD synthetic diamond using apreferred variant of the Tagging Viewer. The stone is 0.80 ct, F/Gcolour and VS1, the bulk of which shows uniform 575 nm PL except for alayer showing blue phosphorescence about ⅓ the way up the pavilionfacets from the culet. Each image is taken looking normal to the tablefacet. On the left is shown the image of 575 nm PL, where the intensityin the original image before conversion to B/W (black and white) wasorange and now shows as lightness or white on black. One the right isshown the image of blue phosphorescence, where the intensity in theoriginal image before conversion to B/W was blue and now shows aslightness or white on black. The intensity in the left hand 575 nm PLimage is generally relatively uniform except for a dark ring seen in thecentre of the table facet. This corresponds to the bright ring seen inthe table in the right hand image of blue phosphorescence. Segments ofthis bright ring can also be seen in the table facets

FIG. 19

Images taken of a square cut CVD synthetic diamond using a preferredvariant of the Tagging Viewer. The stone is 0.69 ct, E/F colour and VVS2or better, the bulk of which shows uniform 575 nm PL except for a layershowing blue phosphorescence about ⅓ the way up the pavilion facets fromthe culet. Each image is taken looking normal to the table facet. On theleft is shown the image of 575 nm PL, where the intensity in theoriginal image before conversion to B/W (black and white) was orange andnow shows as lightness or white on black. One the right is shown theimage of blue phosphorescence, where the intensity in the original imagebefore conversion to B/W was blue and now shows as lightness or white onblack.

The orange luminescence (PL), originating substantially from the 575 nmcentre, extends from around 500 nm to 750 nm at room temperature. (A 575nm PL spectrum as viewed through a band pass filter cutting off below550 nm is shown in FIG. 1). The blue PL/phosphorescence extends from 400nm to 700 nm as shown in FIG. 2. Experiments have shown that wavelengthsless than about 300 nm can excite both the orange 575 nm band (FIG. 3shows the excitation spectra for the 575 nm and 637 nm bands up to theintrinsic diamond band edge (defining the diamond band gap) and is takenfrom Zaitsev A., Optical Properties of Diamond: a data handbook,Springer, 2001 (ISBN 354066582X)) and a weak blue PL/phosphorescenceband. (N.B. 300 nm is outside the spectral response of the eye).

Wavelengths between 227 nm and about 254 nm are most efficient atexciting blue PL/phosphorescence from within the volume of the diamond.It is important to note that excitation at wavelengths less than 227 nmwill be substantially absorbed at the surface of the diamond and willonly generate luminescence at the surface. (This is the method employedin the DiamondView™ instrument. While this would be useful for lookingat tagging layers at the surface, if they are covered by a jewellerysetting where they come to the surface, then a surface excitation methodusing wavelengths less than 227 nm is inappropriate). Since the orange575 nm PL turns off with the excitation source, the perceived colour ofthe PL from the CVD diamond material, or the coloured layers within it,would change from orange, or some orange/blue combination, to blue (theorange/blue flash). The visibility of such a change in colour,particularly the visibility of the orange component against the bluephosphorescence, may need to be enhanced by use of suitable filters.

FIG. 20 shows the spectral decomposition of UV/visible absorptionspectrum of an orangish brown CVD diamond layer, representing a typicalCVD diamond layer grown in the presence of nitrogen without applying themethod of this invention. Spectrum A shows a type lb HPHT syntheticdiamond, spectrum B shows an original spectrum of orangish brown CVDdiamond, spectrum C shows a spectral component with (wavelength)⁻³dependence, and spectrum D shows a spectral component composed of twobroad absorption bands;

FIG. 21 shows a photoluminescence spectrum of a silicon doped CVDdiamond sample recorded at 77 K with 785 nm laser excitation; and

FIG. 22 shows a low magnification optical microscopy image of a sampledescribed in Example 11.

Examples of apparatus that can be used to excite and detect theorange/blue flash effect from 575 nm and or the optical effect from asingle layer are set out below.

APPARATUS EXAMPLE 1 Filtered Xenon Flash Lamp Excitation

The apparatus required to observe the orange/blue flash effect couldconsist of a single excitation source such as a xenon flash lamp. Toreduce the cost of the tagging viewer a low power xenon flash lamp issuggested, although this does not exclude the use of a more powerfulxenon flash lamp. The frequency of the PL would follow the flash lampexcitation frequency. As the PL, and possibly a component of the directexcitation, would be viewed by an observer, the choice of repetitionfrequency is important. Some low power xenon flash lamps will onlydeliver full power at a repetition rate of about 10 Hertz. These arebest avoided on grounds of safety as it is known that flashing light cantrigger epileptic seizure in susceptible individuals. The frequency offlashing light that is most likely to trigger a seizure varies fromperson to person but is generally between 5 and 30 Hertz. A suitablesource could be a 5 Watt xenon flash lamp from Hamamatsu Photonics, typeL9456, operating at a peak power flash repetition rate of about 126Hertz.

The main curve extending across the plot in FIG. 4 shows the emissionspectrum of a Hamamatsu Xenon flash lamp. The radiation from the xenonflash lamp is dominated by intense visible emission bands in the longwave range 400 nm to 550 nm and very intense short wave ultravioletemission bands in the range 220 nm to 270 nm. These long wave and shortwave bands conveniently cover the wavelengths of excitation for the 575nm and blue bands respectively. A suitable off-the-shelf filter incombination with a xenon flash lamp for exciting only 575 nm PL (and noblue phosphorescence) could be a narrow bandpass filter supplied by LOTOriel, UK and manufactured by Andover Corporation, USA, type 450 FS40-25, with a peak wavelength centred at 450 nm, full bandwidth at halfmaximum of about 40 nm and diameter of 25 mm (see FIG. 4). Thiswavelength band is within the vibronic absorption band of the 575 nmcentre. This excitation is also within the vibronic absorption band ofthe 637 nm centre (see FIG. 3). If present the 637 nm luminescence wouldalso usefully be excited with the 575 nm luminescence. In order to viewthe 575 nm luminescence the 450 nm visible excitation must be blocked.This may be carried out effectively with an OG550 glass filter fromComar Instruments, UK. FIG. 1 shows three curves, the curve centredabout 450 nm which is the excitation beam generated by the Xenon flashlamp after filtering by the Andover 450 nm filter described above, therising edge of the band pass region of the OG 550 viewing filter atabout 550 nm, used to remove any of the excitation frequencies from theviewed image, and the emission spectrum of the 575 nm PL centre asviewed through the OG550 filter.

A suitable off-the-shelf filter in combination with a xenon flash lampfor exciting blue luminescence/phosphorescence could be a narrowbandpass filter supplied by LOT Oriel, UK and manufactured by AndoverCorporation, USA, type 228 FS 25-25, with a peak wavelength centred at228 nm, full bandwidth at half maximum of about 25 nm and diameter of 25mm or a similar filter type 232 FS 25-25 with a peak wavelength centredat 232 nm and full bandwidth at half maximum of about 25 nm. FIG. 2shows two curves, the curve centred about 228 nm which is the excitationbeam generated by the Xenon flash lamp after filtering by the Andover228 nm filter and the emission spectrum of the blue PL/phosphorescencecentre. As the excitation does not lie in the visible spectrum novisible blocking filter is required to observe the resultingluminescence and especially not the phosphorescence when the source isswitched off. However, direct viewing of short wave ultraviolet light isextremely harmful to the eyes and must be avoided. Use should be made ofa glass or Perspex-type window to block all harmful ultraviolet lightfrom the observer but allow unimpeded observation of thePL/phosphorescence.

The apparatus described in this example is shown schematically in FIGS.5 and 6. The apparatus labeled ‘575 nm orange fluor.’ shows the viewerset to excite and view 575 nm PL, and relates to the spectra shown inFIG. 1 and described above. The source is the xenon flash lamp. FilterF1 is the 450 nm excitation filter and filter F3 is the orange viewingfilter OG550 and relates to the spectrum shown in FIG. 1 and describedabove. Filter F4 could be an additional filter to reduce effects fromscattered light or the xenon excitation. The apparatus labeled‘blue-green phos.’ shows the viewer set to excite and view bluePL/phosphorescence, and relates to the spectrum shown in FIG. 2 anddescribed above. The source is the xenon flash lamp. Filter F2 is the228 nm excitation filter. To prevent harmful ultraviolet light reachingthe operator a glass or Perspex filter could be placed in the positionmarked ‘open’. FIG. 6 shows the apparatus in side elevation. The viewingfilters are placed at approximately 45 degrees from the vertical and setaway from direct excitation to prevent the operator viewing the sourcedirectly and to eliminate the production of luminescence in the filtersfrom the excitation source. Note that the loose stone under test inFIGS. 5 and 6 has been oriented roughly with its table facing theexcitation source with the observer viewing the stone from the pavilion.In this example provided the stone is fully illuminated the orientationof the stone is unimportant.

APPARATUS EXAMPLE 2 Gas Discharge Lamp Excitation

As previously mentioned, 575 nm luminescence may be excited in theabsence of blue luminescence by wavelengths in the range about 300 nm to575 nm (see FIG. 3). Bulk blue phosphorescence may be excited bywavelengths in the range 227 nm to 300 nm, but wavelengths in the range227 nm to 254 nm are more efficient.

An alternative to using a filtered broadband source to stimulate therespective excitation bands is to employ a dual wavelength laserexcitation. However, in order to remove the necessity for laser safetyrequirements and to reduce the component costs of the tagging viewer theemission from low pressure gas discharge lamps can be used. Inparticular the 254 nm (short wave) and 365 nm (long wave) emissions fromthe mercury discharge lamp can be employed. Mercury long wave and shortwave excitations are used commonly for the observation of luminescencefrom minerals, including diamond. The unique aspect of the presentmethod, as discussed above, is to use the 365 nm excitation first toexcite exclusively 575 nm luminescence and then the 254 nm excitation toexcite the blue phosphorescence. The intense 254 nm line is the dominantemission from low pressure mercury discharge lamps although there areother minor discharge lines (e.g. 365 nm) and broad background dischargein the visible. To minimize the effect of the visible discharge on theobservation of PL in minerals etc the manufacturers of mercury dischargelamps fit a UG5 type filter in front of the lamp. The UG5 filtertransmits below 420 nm and above 650 nm. The 254 nm spectrum in FIG. 7is the mercury discharge through a UG5 filter. The 365 nm emission lampis not purely the result of a mercury discharge. It is produced by the254 nm discharge exciting luminescence from a phosphor coating on theinterior wall of the tube. The 365 nm spectrum is a band with a fullwidth at half maximum of around 25 nm (see FIG. 7). This is veryefficient at exciting 575 nm luminescence but does not excite 637 nmemission from diamond.

Thus, in accordance with the proposed method, the 365 nm source is firstswitched on to excite 575 nm PL in the CVD synthetic. Viewing the 575 nmPL through an OG550 filter is an advantage as the filter removessubstantially all of the background discharge from the 365 nm mercurylamp. The 365 nm lamp should then be switched off and the 254 nm lampswitched on. (The action of switching on the 254 nm lamp could usefullyautomatically switch off the 365 nm lamp). After several seconds the 254nm lamp should be switched off and the presence of blue phosphorescencenoted.

This method could be embodied in the apparatus shown in FIGS. 5 and 6,the xenon flash lamp being replaced by the 254 nm and 365 nm mercurylamps. It has been found that mercury discharge lamps can be arrangedabove the sample under test. In this way a bank of 2, 3, 4, 5 or more254 nm and 365 nm lamps may be mounted together to increase theradiation intensity on the sample. Discharge lamps may be formed intoany shape and could be formed in such a way as to allow circularillumination of the sample allowing almost direct close proximityexcitation from above. The sample could be viewed through the ringillumination. Suitably intense miniature Pen-Ray® lamps supplied byUltra-Violet Products (UVP) USA could be used which would also make theapparatus extremely compact. However any gas discharge lamp capable ofproducing sufficiently intense excitation in the ranges 300 nm to about500 nm (for 575 nm PL) and 227 nm to about 254 nm (for bluePL/phosphorescence) would be suitable. Note that, just as in ApparatusExample 1 the loose stone under test in FIGS. 5 and 6 has been orientedroughly with its table facing the excitation source with the observerviewing the stone from the pavilion. In the present example provided thestone is fully illuminated the orientation of the stone is unimportant.

APPARATUS EXAMPLE 3 Combination of Light Emitting Diode (LED) andMercury Discharge Lamp Excitations

As previously mentioned, 575 nm luminescence may be excited in theabsence of blue luminescence by wavelengths in the range about 300 nm to575 nm (see FIG. 3). Bulk blue phosphorescence may be excited bywavelengths in the range 227 nm to 300 nm, but wavelengths in the range227 nm to 254 nm are more efficient.

In accordance with the proposed method, the long wave source that onlyexcites the orange 575 nm PL could be a light emitting diode (LED). Asuitable LED emitting an emission band centred at 400 nm is shown inFIG. 7. The total optical power output from this type of LED isapproximately 1-2 mW. Higher power LEDs are available from, for example,Nichia Corporation, Japan. Further examples are the 365 nm, 375 nm and380 nm UV LEDs from Nichia or the Luxeon Lumiled Star/C royal blue (455nm) LED. Extreme caution must be observed to protect the observer fromthe intense UV radiation from these sources. The optical emission fromsome of these LEDs can exceed 100 mW.

Provided the viewing filter (for example OG550 or OG570) can block theexcitation wavelengths any LED in the range 300 nm to approximately 500nm is suitable. Experiments have shown that an effective means ofensuring minimal overlap of the excitation wavelengths with the PLemission wavelengths is to use a short wavelength pass filter to blockany long wavelength tail of the excitation. By way of example when usingthe 365 nm UV LED an effective blocking filter is a UG11 or a BG25. Whenusing the 375 nm, 380 nm, 400 nm UV LEDs or 455 nm LED an effectiveblocking filter is a BG25. By way of example when the BG25 and OG550 areboth placed over the UV LED almost total blocking of the LED emission isobtained. Any minor bleed-through from the BG25 can be avoided by anygeometric arrangement that avoids the observer looking directly at theexcitation source through the OG550 filter when observing the PL fromthe sample. By way of example FIG. 8 shows the 400 nm LED excitationband with the transmission curve of the BG25 filter effectivelyextending from 310-520 nm. The BG25 effectively blocks backgroundemission from the LED above 520 nm. The 575 nm PL band transmittedthrough an OG550 filter is shown extending from 550-800 nm, and forcompleteness the transmission of the OG550 is shown from 550-800 nm. Asthe long wavelength tail of the excitation from the 400 nm LED at 700 nmis less than 0.01% of its peak intensity at 400 nm then any minorbleed-through transmission from the BG25 between 700-800 nm is notsignificant.

Thus the use of, for example, a 400 nm UV LED and BG25 blocking filterto excite 575 nm PL and the use of an OG550 to view the 575 nm PL is auseful method in its self to identify CVD synthesized diamond exhibiting575 nm PL.

With this configuration the LED is first switched on to excite 575 nm PLin the CVD synthetic diamond material. Viewing the 575 nm PL through anOG550 filter is an advantage as the filter removes substantially all ofthe long wavelength tail of the LED excitation. The LED should then beswitched off and a 254 nm mercury discharge lamp switched on. (Theaction of switching on the 254 nm lamp could usefully automaticallyswitch off the LED). After several seconds the 254 nm lamp should beswitched off and the presence of blue phosphorescence noted.

This method could be embodied in the apparatus shown in FIGS. 5 and 6,the xenon flash lamp being replaced by the 254 nm mercury discharge lampand LED. It has been found that mercury discharge lamps and LEDs can bearranged above the sample under test. In this way a bank of 2, 3, 4, 5or more 254 nm lamps and a bank of 2, 3, 4, 5 or more LEDs may bemounted together to increase the radiation intensity on the sample. Inthe same way as the mercury discharge lamp may be formed into a circularilluminator, so too could a bank of LEDs be arranged in the form of aring illuminator which could be concentric with the mercury dischargeilluminator. This arrangement would make the apparatus extremelycompact. Note that, just as in Apparatus Examples 1 and 2 the loosestone under test in FIGS. 5 and 6 has been oriented roughly with itstable facing the excitation source with the observer viewing the stonefrom the pavilion. In the present example provided the stone is fullyilluminated the orientation of the stone is unimportant.

APPARATUS EXAMPLE 4 Optical Effect from Luminescent Laver

The tagging layer should be positioned so that in any mount not normallyeasily removed, such as a jewellery setting, the volume of the layer canbe efficiently excited by an external light source used duringidentification, the key point being that this light distribution maydiffer from that in normal viewing conditions, being for example a highintensity parallel beam rather than a more diffuse light source. Anotherexample may be a high intensity ring illuminator placed over the sample.The interaction between the light source and the target cut stone issensitive to the geometry or cut of the stone, and for precise analysisneeds advanced ray tracing calculations. More importantly, theinteraction between the luminescence or phosphorescence emitted by alayer or region within the cut stone and the cut of that stone, whichforms the pattern of rays seen by the observer is sensitive to thegeometry or cut of the stone, and the position of the layer or regionwithin it, and for precise analysis needs advanced ray tracingcalculations. Such ray tracing calculations have been carried out.

Considering a nitrogen doped layer (containing 575 nm/637 nm centres)extending about 13% of the height of the stone from the culet/point ofthe stone, the efficient excitation of the layer may require carefulcontrol of the excitation beam angle but this positioning of the layermay then be more effective in providing emission of internalluminescence directly out through the table.

FIG. 9 a (lower left) shows an apparatus consisting of a ringilluminator with excitation near the normal of the table of the stone.The illumination is being used to solely excite 575 nm PL. Theillumination could be a 365 nm mercury discharge lamp, a filtered xenonflash lamp, a 365 nm, 375 nm, 380 nm, 400 nm or 455 nm LED or anysuitably filtered intense source that excites 575 nm PL. The ray tracingdiagram in FIG. 9 a has been produced from a CVD synthetic 6 mm highwith a 575 nm containing region/layer extending 0.8 mm (13% the heightof the stone) from the culet. The eye (3 mm diameter pupil) of theobserver is about 100 mm from the culet. The ray tracing diagram wasproduced by generating 4 million rays from within the 575 nm containinglayer and calculating their trajectory within the CVD synthetic andtheir exit points. Only those rays (around 800) that entered the 3 mmaperture have been plotted on the plan view of the stone. An orange spotis seen in high contrast clearly evident in the centre of the table. Therays responsible for this spot exited the stone within the 22.4 degreescritical angle and therefore without internal reflection. Rays from thelayer incident on the table facet outside the critical angle undergointernal reflection and exit at the crown facets with a distributionshown in FIG. 9 a. This production of a coloured (in this case orange)spot is likely to be unique to a CVD synthetic with a well definedvolume or layer producing luminescence or phosphorescence (in this casea nitrogen doped layer producing 575 nm PL) placed near the culet.

FIG. 10 shows a preferred apparatus 100 to enable an observer to see theorange/blue flash and the optical effect from an orange or blueluminescent layer in a tagged CVD synthetic. As previously mentioned,bulk 575 nm luminescence may be excited in the absence of bluePL/phosphorescence by wavelengths in the range about 300 nm to 575 nm(see FIG. 3). In accordance with the preferred apparatus 100, the longwave source 102 that only excites the orange 575 nm PL could be a LuxeonLumiled Star/C royal blue light emitting diode (LED) emitting about 150mW of optical radiation at 455 nm. When using the 455 nm LED aneffective blocking filter 104 to prevent the observer seeing theexcitation radiation is BG25. The BG25 filter effectively blocks lightabove 520 nm.

The 575 nm orange PL band excited in the CVD stone 106, mounted in aring/stone mount 108, by the 455 nm LED is observed with high puritythrough an OG550 filter 110.

As previously mentioned, bulk blue phosphorescence may be excited bywavelengths in the range 227 nm to 300 nm, but wavelengths in the range227 nm to 254 nm are more efficient. In accordance with the preferredapparatus, the short wave source 112 that excites the bluePL/phosphorescence is a 5 Watt xenon flash lamp from HamamatsuPhotonics, type L9456-01, operating at a peak power flash repetitionrate of 126 Hertz. A suitable off-the-shelf filter 114 for transmittingonly the deep UV excitation from the lamp (and exciting the blueluminescence/ phosphorescence) is the type 232 FS 25-25 narrow bandpassfilter supplied by LOT Oriel, UK and manufactured by AndoverCorporation, USA, with a peak wavelength centred at 232 nm and fullbandwidth at half maximum of about 25 nm.

The complete apparatus is contained within a darkened box 116 to allowthe luminescence to be observed without interference from stray light. Aviewing lens 118 within the apparatus allows the observer to see amagnified image (×2.5 for example) of the stone. The viewer was designedfor use in a retail environment so great care was taken to protect theobserver from harmful UV using a glass or Perspex viewing window 120located below a viewing visor 122. The tagging viewer was also designedto sit on a microscope stage (not shown), and the fine spatialdistribution of the luminescence/phosphorescence emitted by the diamondmaterial or body under test was easily discernable and recorded with adigital camera.

With this configuration the LED is first switched on by continuallydepressing the LED 102 button to excite 575 nm/orange PL in the CVDsynthetic 106. The orange PL is observed through the OG550 filter 110.If there is a nitrogen doped layer producing 575 nm PL below the girdleof the stone then the observer will see a distinctive orange ring orspot in the centre of the table facet as illustrated by the ray tracediagram in FIG. 9 a.

This production of an orange spot is likely to be unique to a CVDsynthetic. When the LED 102 button is released the orange PL ceases. TheOG550 filter 110 is then manually removed. The xenon flash lamp 112 isthen switched on by continually depressing the UV lamp button. Theobserver will then see a blue PL image of the CVD synthetic 106 withpossibly some orange PL. When the UV button is released the observerwill see blue phosphorescence. If there is a boron doped layer below thegirdle of the stone then the observer will see a distinctive blue ringor spot in the centre of the table facet in both PL and phosphorescenceas illustrated by the ray trace diagram in FIG. 9 a. This production ofa blue spot is likely to be unique to a CVD synthetic.

An alternative to a layer forming a portion of the cut stone up from theculet to a single boundary, is a discrete layer. Using the apparatusjust described, shown in FIG. 17 are two images of a single tagged CVDsynthetic polished into a 0.80 carat round brilliant. The bulk of thestone was grown with nitrogen. Under 455 nm LED excitation the bulk ofthe stone produces orange 575 nm PL as shown in the image on the left ofFIG. 17. In order to produce an image of a discrete layer clearlyvisible in the table of the cut stone, a preferred layer was positionedwell below the girdle, (typically) about ¼-⅓ up the height of the cutstone. Thus the stone in FIG. 17 has a boron doped layer phosphorescent,around 200-300 μm thick, at this position. This discrete layer onlyproduces very weak 575 nm PL and therefore provides a reasonably welldefined dark ring which is visible when viewing the 575 nm PL producedthrough the table. However, when the LED is switched off and the xenonflash lamp switched on, the dark ring corresponding to the boron dopedlayer, is rendered highly visible as a well defined intense bluePL/phosphorescence ring viewed through the table as seen in the righthand image in FIG. 17.

Additional images of segments of this ring may be visible in the crownfacets, dependent on the exact orientation of the facets, with theposition of the segment of the ring being similar in each facet of thesame type and angle, but varying between the facets of different typeand angle, to provide a complex series of features reflecting thesymmetry of the stone. The production of a well defined coloured ring(in this case a blue ring) viewed within the table is likely to beunique to a CVD synthetic with a well defined volume or layer producingluminescence or phosphorescence (in this case a boron doped layer)placed in the lower half of the cut stone, below and away from thegirdle. Indeed, this case is clearly represented by the right hand imageof the example stone in FIG. 17.

FIG. 9 b shows a ray tracing diagram for a stone with, for example, a575 nm PL layer just below the girdle. The effect this time is toproduce a well defined orange ring just outside the table facet of theCVD synthetic. Similarly, the production of a well defined coloured ring(in this case an orange ring) is likely to be unique to a CVD syntheticwith a well defined volume or layer producing luminescence orphosphorescence (in this case a nitrogen doped layer producing 575 nmPL) placed just under the girdle. Those skilled in the art willunderstand that a range of other positions of the layer are possible,with the perceived pattern encompassing a variety of rings and otheridentifying patterns, but that the key feature is that the non-naturaldistribution of optical centres in the cut diamond is detectable as anon-natural pattern of colour in the viewed stone, preferably viewedfrom the table, under suitable conditions.

By way of further example FIG. 11 shows a PL image of a 0.2 carat roundbrilliant CVD synthetic 10 (crown angle approximately 35 degrees to theplane of the girdle and pavilion angle 41.5 degrees to the plane of thegirdle) recorded by the DiamondView™ instrument. The CVD synthetic 10has a blue PL layer extending from the culet point to approximately 30%of the height of the stone. The image was recorded with the table facet12 facing the viewer. The DiamondView™ excitation is sufficientlypenetrating in a type II CVD synthetic to excite substantial sub-surfacePL. The image of the blue PL layer in the DiamondView™ is very similarto that observed simply by eye with bulk excitation of the stone withshort wave UV light with wavelengths in the range 227 nm toapproximately 254 nm. The image is dominated by a distinct blue‘fish-eye’ spot 14 in the centre of the table facet 12. The remainder ofthe table facet 12 is devoid of blue PL. The crown facets 16 show adistribution of intensity from the internally scattered blue PL.

FIG. 11 compares very well with an image from a stone 20, with similargeometry, generated using the ray tracing programme for a PL layer 22extending approximately 30% the height of the stone 20 from the culetpoint 24, shown in FIG. 12. Like FIG. 11, FIG. 12 is also characterizedby a ‘fish-eye’ spot 26 in the centre of the table facet 28, theremainder of the table facet 28 being devoid of PL. As in the case ofthe DiamondView™ image and Tagging viewer image, the crown facets 30show a distribution of intensity from the internally scattered PL.

It should be noted that the stone 10 used in FIG. 11 and the stone usedin FIG. 17 also have an orange 575 nm layer above the blue PL layer.Thus these stones will also perfectly demonstrate the orange/blue flasheffect in the DiamondView™ and the Tagging viewer and using the methodsdescribed above.

There is an effect on the observed image from a variation in thepavilion angle. However, the resulting images are sufficiently welldefined to provide great confidence in the use of the optical effectfrom luminescent layers for stones of varying pavilion and crown anglesand stone shapes such as square cut or emerald cut. The effect on the PLimage of changing the pavilion angle in a round brilliant stone from41.5 degrees (FIG. 12) to 25 degrees in a stone 40 is shown in FIG. 13.The characteristic ‘fish-eye’ 42 is clearly visible and thereforeidentifies the stone as a tagged CVD synthetic.

Particularly in the case where the layers are intended to form a uniquesignature representing the manufacturer or other information, ratherthan just provide evidence of the synthetic nature of the material, thenconsideration needs to be given to the structural sequence of thelayers. The characteristic pattern of lines forming a tag within a cutCVD synthetic stone needs to be as widely applicable as possible, andpotentially could be used in CVD diamond layers which are themselvesboron doped to provide a visible blue colour, but also providing asource of blue-band phosphorescence, or in CVD diamond layers or objectswhich otherwise contain nitrogen and thus show orange luminescencethrough part or the whole of their volume. By combining the two layerstogether into a pattern, the same pattern can then be observed andidentified in both these types of CVD diamond layer or object, with therisk that final layers at the edge of the mark may blend into thebackground. This risk is minimised by using an asymmetric mark, whereone edge of the mark is defined by blue phosphorescence and the other isdefined by orange luminescence. Alternatively some neutral backgroundcould be used around the mark, or the mark could be deliberately variedat the pattern edges to provide clarity in any particular type of stone

In CVD diamond layers with a sufficiently high level of boron moregenerally present through the volume, for example where a stronglycoloured stone is required, then the addition of nitrogen in markerlayers may not be sufficient to generate orange luminescence but maymerely modulate the blue phosphorescence from donor-acceptor pairrecombination. One solution is to specifically reduce the Bconcentration in the N doped layers to enable the orange luminescence tobe observed. Alternatively, under such circumstances the modulation ofthe blue phosphorescence may be sufficient, and can be controlled byboth the added N and added B concentrations.

Likewise, in a process where nitrogen is added for other reasons and 575nm PL is present throughout the layer, an alternative to producing 575nm luminescing layers is to produce layers free of 575 nm luminescence,or to modulate the intensity of the 575 nm luminescence by varying thenitrogen concentration or by other process variations such as themethane concentration or the temperature.

There are further advantages to the choice of marker layers, in that theblue phosphorescence of the boron is stable with respect to posttreatments such as annealing, so that these marks would remain even ifthe CVD diamond layer, object or synthetic gemstone was treated by suchmeans. Conversely the orange luminescence is modified by annealing,particularly at very high temperatures. These lines would thereforeindicate that the object had been post treated after the point of sale.In particular, annealing of the orange luminescence can convert theorange luminescence to a characteristic green luminescence orphosphorescence (the extent to which this light continues to be emittedafter the removal of the excitation source varying over several ordersof magnitude depending on the relative concentration of the defectsinvolved). The stability of the layers showing blue phosphorescence thusenables the location of the previously orange luminescing bands to bedetermined and the treatment conditions to be determined from themodified, increased or reduced colour then present in these bands.

As will be described in the example sections, a distinctive orange/blueflash is observed from tagged CVD synthetics under suitable illuminationconditions. In annealed tagged CVD synthetics (particularly thoseannealed at very high temperatures) the orange/blue flash may bereplaced with a green/blue flash. This effect would be noticeable, by asuitably trained individual, from a tagged CVD synthetic cut into anyshaped brilliant e.g. round or square.

As described earlier and further described by the examples, adistinctive orange/blue ‘fish-eye’ ring or spot is observed fromsuitably tagged CVD synthetics under suitable illumination conditions.As the blue phosphorescence of the boron doped CVD diamond is stablewith respect to post treatments, such as annealing, the blue ‘fish-eye’ring (picture-frame in square cut stones) or spot remains unaltered andis still an effective means of identifying a tagged CVD synthetic stoneeven after annealing in both round cut and square cut stones.

A particular variant of the invention is the deliberate use of the greenluminescence obtained after annealing orange luminescing material,either in isolation or in combination with other tagging centres andstructures.

The simplest pattern of multiple lines envisaged using one type ofmarker layer (e.g blue phosphorescence) is shown in FIG. 14. The diamondmaterial 50 includes a pair of marker layers 52,54 separated by a spacerlayer 56.

Here, t_(m) is the thickness of the respective marker layers 52,54 andt_(s)is the thickness of the spacer layer 56.

Adding in a second type of layer (e.g. orange luminescence) gives astructure as shown in FIG. 15. In this embodiment, the synthetic diamondmaterial 60 has a pair of first marker layers 62,64 separated by asecond marker layer 66. A simpler structure which may be moreappropriate in some circumstances is one layer of each of the two typesof marker layer placed adjacent to one another, with a further variantbeing where these two layers are spaced apart by a spacer layer ofundoped or untagged material.

In particularly preferred embodiments, it is envisaged that thestructures would probably be more complex, using more layers and withclearly varying thicknesses. In the synthetic diamond material 70 shownin FIG. 16, the thicknesses of the layers 72, 74, 76, 78, 80 and 82 arevaried. They may be, for example, 50 μm (74,80), 25 μm (76,78), and 12μm (72,82), giving a total thickness of the mark of 175 μm, which wouldbe clearly visible under the correct viewing illumination provided thatthe dopant levels were suitably controlled.

It has been demonstrated that the marker layers of given thicknesses canbe grown to an accuracy of 10% or better, with typical values being inthe 3%-5% region. For thicker layers or in a routine production processit may be possible to achieve an accuracy of 2% or better. However, whenviewing the layers with above band-gap illumination, the marker layerswill be seen on any surface which intersects those layers, but thatsurface may not be normal (i.e. at right angles) to the marker layers,and in many commercial diamond objects including synthetic gemstonesthis will often be the case. Thus, the absolute dimensions of the layerswill typically not be consistent from one facet of a CVD diamond objectto another, or necessarily between similar objects (although this may bethe case if both the marker layers and the orientation of the object cutout of the diamond are crystallographically oriented), easily varying byup to about +/−50% depending on the angle of the facet on which they areviewed. What will be consistent across any single facet, however, is therelative ratios of the thicknesses of the layers and the sequence ofcolours, which will allow for proper identification of the mark oforigin or fingerprint. It is of course possible from the geometry of theCVD diamond object and the specific orientations of the layer(s) and thefacet intersected, to calculate the precise thicknesses of the layers,but this is a level of complexity which the preferred embodiment avoids.It may also be possible to measure these thicknesses directly usingtechniques such as confocal depth profiling, but again this generallyneeds more complex equipment than is desirable.

Thus, taking relative ratios of the thicknesses of layers as the onlymeasurable characteristic, a single marker layer gives no information,since there is no reference point. However, a mark of origin with 2marker layers and a spacer layer gives two unique parameters, forexample taking the spacer layer as the scale bar against which tocompare the thickness of each of the marker layers. A mark of originwith 3 marker layers and 2 spacer layers gives 4 unique parameters(provided there is no mirror symmetry), etc. Hence, in practice, it isbelieved that the reasonable number of layers to provide adistinguishable mark of origin but permitting several deliberatevariants would be three marker layers, giving 4 unique thickness ratioparameters. In the case where 2 distinct type of marker layers are usedalternately, the number of unique parameters can be considered in asimilar fashion.

By way of example the invention is described above giving detail ofspecific detail of optical centres and layered structures which areadvantageous. However, those skilled in the art will understand thatthis does not limit the generality of the invention, which in its mostgeneral form provides a means of detecting the synthetic nature of adiamond layer without affecting its visual properties under normalviewing conditions. A preferred form is the use of optical centres suchas the 575 nm PL centre to provide the synthetic indicators. However, itmay be possible to use other features or properties of the material. Afurther preferred form is the use of layered structures to emphasise thedeliberate synthetic nature of the material. A particularly preferredform is the combination of the use of optical centres and layeredstructures to provide clear evidence of the synthetic nature of thematerial even where access or other considerations may add difficulties.

The invention will now be described with reference to the followingnon-limiting Examples.

EXAMPLE 1

Substrates suitable for synthesising single crystal CVD diamond wereprepared according to the method described in WO 01/96634, with {100}major faces.

These substrates were brazed onto a tungsten substrate using a hightemperature diamond braze. This was introduced into a microwave plasmaCVD reactor and an etch and growth cycle commenced in the general formdescribed in WO 01/96634, and then synthesis proceeded as follows:

The first stage of growth comprised 200/250/4500 sccm (standard cubiccentimetre per second) of CH₄/Ar/H₂ at 200×10² Pa and a substratetemperature of 850° C. with no added dopants.

The second stage of growth was the same as the first stage above withthe addition of 0.8 sccm of 20 ppm B₂H₆ diluted in hydrogen (0.003 ppm),and the addition of 25 sccm of 100 ppm N₂ diluted in hydrogen (0.5 ppm).

The third stage of growth was the same as the first stage above with theaddition of 10 sccm of 100 ppm N₂ diluted in hydrogen (0.2 ppm).

The fourth stage was a repeat of the first stage.

On completion of the growth period, the substrate was removed from thereactor and the CVD diamond layer removed from the substrate. This layerwas then polished to produce a 6.7×6.6×2.3 mm diamond block of {100}growth sector material and analysed for its optical properties and thestructure of the layers.

Using above bandgap radiation in a ‘DiamondView™’ the structure of thelayers was determined by viewing a side face of the block to be: layer1: 450 μm thick, layer 2: 250 μm thick, layer 3: 285 μm thick and layer4: 1.31 mm. Layer 2 showed strong phosphorescence and layer 3 showedstrong 575 nm luminescence. This layer structure is unique to syntheticdiamond of this invention

Under a standard jewelers UV hand lamp it was possible to discernluminescence and phosphorescence from the stone in a darkened room,although the blue phosphorescence tended to dominate the orangeluminescence during exposure.

A low cost viewer suitable for volume production was constructed toevaluate the phosphorescence and luminescence properties of the diamonddescribed in detail in Apparatus Example 4 and illustrated in FIG. 10.The viewer comprised a 5 W OEM pulsed Xenon unit (Hamamatsu Photonics,type L9456) and a Luxeon Lumiled Star/C LED emitting at 455 nm.

A 0.2 ct round brilliant cut synthetic was produced from a similar blockof CVD diamond produced in the same synthesis run and was graded to be Hcolour. The first layer, below the boron phosphorescent layers, wasremoved during processing. The appearance of the 575 nm luminescence andblue phosphorescence observed in this stone when viewed through thetable in DiamondView™ and Tagging viewer was described earlier (FIG. 11and FIG. 12), with a distinct blue ‘fish-eye’ spot visible in the centreof the table surrounded by orange luminescence, and a distinct patternof blue phosphorescence and orange luminescence visible in the crownfacets.

EXAMPLE 2

The growth procedure described in Example 1 was repeated to produce alayer 5×5×3 mm thick.

Vertical plates were cut out of this block and fabricated into diamondscalpel blades. The presence of the tagging layers was not discernableunder normal illumination in these blades and did not affect theirnormal function.

Upon inspection using DiamondView™, the presence and structure of thetagging layers was clearly discernible, identifying the origin of thematerial from which the blades were fabricated.

Inspection under the low cost viewer described in Apparatus Example 4,and Example 1 and illustrated in FIG. 10 clearly showed orangeluminescence and blue phosphorescence, making clear the unique syntheticnature of the material.

EXAMPLE 3

The growth procedure described in Example 1 was repeated to produce alayer 3.7 mm thick. This layer was polished into a round brilliant cut.

Upon inspection using DiamondView™, the presence and structure of thetagging layers was clearly discernible, cutting across the facets justbelow the girdle, identifying the origin of the material from which thestone was produced.

Inspection under the low cost Tagging viewer described in Example 1,Apparatus Example 4 and illustrated in FIG. 10 clearly showed orangeluminescence and blue phosphorescence, making clear the unique syntheticnature of the material.

EXAMPLE 4

Using growth conditions similar to Example 1, but varying the durationof the different layers, a series of demonstrator stones in the form ofround brilliants and square cut stones have been produced. Thedemonstrator stone images are shown in FIGS. 17 to 19. The left handimage in each figure is the image of the stone under 455 nm LEDexcitation and showing 575 nm/orange PL. The right hand image in eachfigure is the image of the stone under 232 nm deep UV excitation fromthe filtered xenon flash lamp and shows the blue PL/phosphorescence.FIGS. 17 and 18 show round brilliant demonstrator stones and FIG. 19shows a square cut demonstrator stone.

EXAMPLE 5

Ib HPHT substrates suitable for synthesising single crystal CVD diamondwere prepared according to the method described in WO 01/96634, with{100} major faces.

These substrates were brazed onto a tungsten substrate using a hightemperature diamond braze material. This was introduced into a microwaveplasma CVD reactor and an etch and growth cycle commenced in the generalform described in WO 01/96634, but using the specific synthesisconditions described below.

Nitrogen was added into the process using a mixture of 100 ppm N₂ inhydrogen. Boron impurities were added to the process using either 20 ppmor 100 ppm B₂H₆ in hydrogen.

Two sets of samples were prepared.

Sample Set 1-1

The first stage of growth comprised 200/250/4500 sccm (standard cubiccentimetre per minute) of CH₄/Ar/H₂ at 200×10² Pa and a substratetemperature of 850° C. with no added dopants. This was a control layerof high purity high colour growth to demonstrate process control.

The second stage of growth was the same as the first stage above withthe addition of 1 ppm of N₂. This stage was to evaluate the effect of1.0 ppm nitrogen as the sole gaseous impurity.

Sample Set 1-2

The first stage of growth repeated the conditions for the first stage ofgrowth for set 1-1.

The second stage of growth was the same as the first stage above withthe addition of 0.003 ppm B₂H₆, and the addition of 1 ppm N₂.

On completion of the growth period, the samples where removed from thereactor and processed to produce a range of test pieces, in particularcross-sectional slices of the growth, and free standing plates of thesecond stage growth layer which were typically 2-3 mm thick. Thecross-sectional slices confirmed that the first stage growth in eachcase was essentially colourless high purity growth, and that whilst thesecond stage growth in sample set 1-1 was significantly coloured brownthe second stage growth in the second process with added boron wasalmost colourless.

A number of further samples were produced, using the general growthconditions of: 200/250/4500 sccm (standard cubic centimetre per minute)of CH₄/Ar/H₂ at 200×10² Pa and a substrate temperature of 850° C., withthe addition of 0.003 ppm B₂H₆, and the addition of 1 ppm N₂, but makingsmall variations to the different growth parameters and in particularvarying the temperature by +/−100° C., the relative ratio of the B and Nconcentration by a factor of 5 in both directions (e.g. higher and lowerboron relative to nitrogen), and the pressure by +/−100×10² Pa. Theconclusion was that too much B relative to nitrogen produced bluematerial, that too little B relative to nitrogen produced brownmaterial, and that the optimum balance between B and N varied to somedegree with other process parameters such as pressure and temperature.However, the deleterious effect of the nitrogen in producing browncoloured or optically absorbing diamond could be ameliorated and largelystopped by adding an optimal level of boron to the process, matched tothe particular growth conditions used.

EXAMPLE 6

Ib HPHT diamond substrates were prepared and mounted onto a tungstendisc as in example 5. This disc was introduced into a microwave plasmaCVD reactor and an etch and growth cycle commenced in the general formdescribed in WO 01/96634, but using the specific synthesis conditionsdescribed below.

Nitrogen was added into the process using a mixture of 100 ppm N₂ inhydrogen. Silicon impurities were added to the process using typically500 ppm SiH₄ in hydrogen.

Two sets of samples were prepared.

Sample Set 2-1

The first stage of growth comprised 36/0/600 sccm (standard cubiccentimetre per minute) of CH₄/Ar/H₂ at 250×10² Pa and a substratetemperature of 810° C. with no added dopants. This was a control layerof high purity high colour growth to demonstrate process control.

The second stage of growth was the same as the first stage above withthe addition of 2.0 ppm of nitrogen. This stage was to evaluate theeffect of 2.0 ppm nitrogen as the sole gaseous impurity.

Sample Set 2-2

The first stage of growth repeated the conditions for the first stage ofgrowth for set 2-1.

The second stage of growth was the same as the first stage above withthe addition of 0.3 ppm of silane, and the addition of 2.0 ppm ofnitrogen.

On completion of the growth period, the samples where removed from thereactor and processed to produce a range of test pieces, in particularcross-sectional slices of the growth, and free standing plates of thesecond stage growth layer which were typically 2-3 mm thick. Thecross-sectional slices confirmed that the first stage growth in eachcase was essentially colourless high purity growth, and that whilst thesecond stage growth in sample set 2-1 was significantly coloured brownthe second stage growth in the second process with added silicon (set2-2) was almost colourless.

A number of further samples were produced, using the general growthconditions of: 36/0/600 sccm (standard cubic centimetre per minute) ofCH₄/Ar/H₂ at 250×10² Pa and a substrate temperature of 810° C., with theaddition of silane in the range 0-5 ppm, and the addition of nitrogen inthe range of 0-10 ppm, and also making small variations to the differentgrowth parameters and in particular varying the temperature by +/−100°C., and the pressure by +/−100×10² Pa. In particular the followingcombinations were tested, recording silane/nitrogen concentrations inppm of: 0.2:1, 1:1, 5:10. In each case the effect of the silane was tosuppress any brown colouration in the diamond which would otherwise havearisen. In addition, excess silicon over that needed to suppress thebrown colouration did not generate any deleterious colour or otherchanges in the diamond growth. This provides an additional advantage ofthe use of Si as a gaseous impurity over that of B in terms of reducingthe deleterious effect on nitrogen, since the concentration of thesilane is not critical, and the level of nitrogen concentration in thegas phase or the precise value of other process parameters becomes muchless important.

EXAMPLE7

Ib HPHT diamond substrates were prepared and mounted onto a tungstendisc as in example 5. This disc was introduced into a microwave plasmaCVD reactor and an etch and growth cycle commenced in the general formdescribed in WO 01/96634, but using the following specific synthesisconditions:

The growth conditions were 36/0/600 sccm (standard cubic centimetre perminute) of CH₄/Ar/H₂ at 250×10² Pa and a substrate temperature of 810°C., with a silane concentration of 0.25 ppm and nitrogen concentrationof 2 ppm. Growth was continued until the thickness of the CVD layers was2 mm. After termination of the growth process the samples were removedand one was processed to produce a free-standing parallel-sided plate ofsingle-crystal CVD diamond. Another was processed to produce a {100}cross-sectional slice. Characterisation of the free-standing CVD plategave the following results:

a) SIMS measurements carried out in four places on each side of theplate indicated a uniform Si concentration of approximately 6×10¹⁵ cm⁻³(34 ppb).

b) UV/visible/NIR absorption spectroscopy measurements carried out atroom temperature indicated that the absorption coefficient was less than0.15 cm⁻¹ for all wavelengths between 300 nm and 1000 nm. The absorptioncoefficient at 270 nm was 0.19 cm⁻¹ and, after baseline subtraction, thepeak absorption coefficient of the 270 nm feature was 0.036 cm⁻¹,indicating a concentration of uncompensated nitrogen of approximately 24ppb. The absorption coefficients at 350 nm and 520 nm were 0.10 and 0.07cm⁻¹; respectively.

c) The CIELAB chromaticity coordinates for a 0.5 ct round brilliantproduced from material of this kind were estimated from the absorptionspectroscopy data using the method described earlier and were found tobe:L*=87.9, a*=−0.13, b*=1.07, C*=1.08Using the method described earlier, it can be deduced that a stone withthese chromaticity coordinates would have an F GIA colour grade.

d) Absorption spectroscopy carried out at 77 K indicated a strong 737 nmfeature with an integrated absorption coefficient of 6.02 meV.cm⁻¹.

e) Photoluminescence spectroscopy carried out with 633 nm excitation at77 K indicated that the Raman normalised intensity of the Si-relatedfeature at 737 nm was 4.

f) Photoluminescence spectroscopy carried out with 514 nm excitation at77 K indicated photoluminescence features at 575, 637 and 737 nm withthe following Raman normalised intensities: Feature Raman normalisedintensity 575 nm 0.05 637 nm 0.03 737 nm 6

g) Photoluminescence spectroscopy carried out with 325 nm excitation at77 K indicated photoluminescence features at 533 nm and 575 nm.

Characterisation of the cross-sectional slice gave the followingresults:

a) SIMS measurements again indicated a Si concentration of approximately6×10¹⁵ cm⁻³ (34 ppb) for the dominant <100> sector. Significantly higherSi concentrations were measured for minor <100> sectors that had beenformed by growth originating at the {100} edge faces of the substrate.In some regions, which optical microscopy indicated werenear-colourless, the Si concentration was found to be higher than 10¹⁸cm⁻³ (5.7 ppm).

b) Photoluminescence spectra collected at 77 K with 633 nm excitationshowed a strong Si-related feature at 737 nm with a Raman normalisedintensity of approximately 4 for the dominant <100> sector and rising toalmost 40 in the minor <100> sectors.

c) Luminescence images of the CVD material, created with above band gapexcitation, were dominated by orange red luminescence.

EXAMPLE 8

Ib HPHT diamond substrates were prepared and mounted onto a tungstendisc as in example 5. This disc was introduced into a microwave plasmaCVD reactor and an etch and growth cycle commenced in the general formdescribed in WO 01/96634, but using the following specific synthesisconditions.

The growth conditions were 36/0/600 sccm (standard cubic centimetre perminute) of CH₄/Ar/H₂ at 250×10² Pa and a substrate temperature of 810°C., with a silane concentration of 0.25 ppm and nitrogen concentrationof 1 ppm. Growth was continued until the thickness of the CVD layers was0.7 mm. After termination of the growth process the samples were removedand one was processed to produce a free-standing parallel-sided plate ofsingle-crystal CVD diamond. Another was processed to produce a {100}cross-sectional slice. Characterisation of the free-standing CVD plategave the following results:

a) SIMS measurements carried out in four places on each side of theplate indicated a uniform Si concentration of approximately 5×10¹⁵ cm⁻³(28 ppb).

b) UV/visible/NIR absorption spectroscopy measurements carried out atroom temperature indicated that the absorption coefficient was less than0.5 cm⁻¹ for all wavelengths between 300 nm and 1000 nm. The absorptioncoefficient at 270 nm was 0.5 cm⁻¹ and, after baseline subtraction, thepeak absorption coefficient of the 270 nm feature was 0.074 cm⁻¹,indicating a concentration of uncompensated nitrogen of approximately 50ppb. The absorption coefficients at 350 nm and 520 nm were 0.32 and 0.28cm⁻¹ respectively.

c) The CIELAB chromaticity coordinates for a 0.5 ct round brilliantproduced from material of this kind were estimated from the absorptionspectroscopy data using the method described earlier and were found tobe:L*=84.0, a*=−0.19, b*=−0.43, C*=0.47Using the method described earlier, it can be deduced that a stone withthese chromaticity coordinates would have an E GIA colour grade.

d) Absorption spectroscopy carried out at 77 K indicated a strong 737 nmfeature with an integrated absorption coefficient of 5.41 meV·cm⁻¹.

e) Photoluminescence spectroscopy carried out with 633 nm excitation at77 K indicated that the Raman normalised intensity of the Si-relatedfeature at 737 nm was approximately 0.90.

f) Photoluminescence spectroscopy carried out with 514 nm excitation at77 K indicated photoluminescence features at 575, 637 and 737 nm withthe following Raman normalised intensities: Feature Raman normalisedintensity 575 nm 0.022 637 nm 0.016 737 nm 2

g) Photoluminescence spectroscopy carried out with 325 nm excitation at77 K indicated photoluminescence features at 533 nm and 575 nm.

Characterisation of the cross-sectional slice gave the followingresults:

a) SIMS measurements again indicated a Si concentration of approximately5×10¹⁵ cm⁻³ (28 ppb) for the dominant <100> sector. Significantly higherSi concentrations were measured for minor <100> sectors that had beenformed by growth originating at the {100} edge faces of the substrate.In some regions, which optical microscopy indicated werenear-colourless, the Si concentration was found to be higher than 10¹⁸cm⁻³ (5.7 ppm).

b) Photoluminescence spectra collected at 77 K with 633 nm excitationshowed a strong Si-related feature at 737 nm with a Raman normalisedintensity of approximately 1 for the dominant <100> sector and rising toapproximately 4 in the minor <100> sectors.

c) Luminescence images of the CVD material created with above band gapexcitation were dominated by orange red luminescence.

EXAMPLE 9

A layered single crystal CVD diamond sample was grown on a {100} HPHTsynthetic substrate in six different stages. The gas flow rates were36/600 sccm (standard cubic centimetres per minute) of CH₄/H₂ and thesubstrate temperature was 810° C. Nitrogen and silane were supplied togive the concentrations in the process gases listed in Table 3 below forthe different stages in the growth process. Growth was terminated whenthe total thickness of CVD growth was 1.4 mm.

A {100} polished cross-sectional slice was processed from the sample toenable the properties of the layers to be studied. When the slice wasviewed under an optical transmission microscope the CVD growth wasuniformly colourless. Distinct layers corresponding to the differentstages of growth could however be clearly distinguished in luminescenceimages of the slice recorded using above band gap excitation. They werealso easily identifiable in cathodoluminescence images recorded using ascanning electron microscope equipped with an Oxford Instruments lowmagnification cathodoluminescence imaging system. A MonoCL spectrometerwas used to measure the intensity of 235 nm free exciton luminescenceemitted by each of the layers under electron beam excitation. Table 3lists the gas phase silicon and nitrogen concentrations, the resultingsilicon concentrations measured using SIMS and the free excitonluminescence intensities measured relative to a standard sample of highpurity CVD diamond. TABLE 3 Si conc. N conc Free exciton CL SilaneNitrogen SIMS SIMS intensity relative to high Layer (ppm) (ppm) (ppm)(ppm) purity standard 1 4.0 0 0.21 <0.5 1.00 2 3.9 2.0 0.24 <0.5 1.00 33.8 3.8 0.4 <0.5 0.94 4 3.8 5.7 0.61 <0.5 0.78 5 3.7 7.4 0.86 <0.5 0.726 4.0 10 3.75 <0.5 0.34

This example demonstrates that the diamond grown can show surprisinglystrong free exciton luminescence (measured relative to that shown by ahigh purity diamond standard) even though it contains significantconcentrations of silicon and is grown in the presence of aconcentration of nitrogen that would normally cause the material to showvery weak free exciton emission. At the highest silicon concentrations,achieved with higher gas phase nitrogen concentrations, the free excitonemission is significantly weaker but the material still has a very lowabsorption coefficient across the visible region of the spectrum and istherefore colourless. The absorption coefficient spectrum was derivedfrom absorbance measurements (after subtraction of the calculatedreflection loss spectrum) and for all positions across the sample theabsorption coefficient in the range 350-800 nm was found to be less than0.9 cm⁻¹ and only at 737 nm did it rise above 0.7 cm⁻¹.

EXAMPLE 10

A layered single crystal CVD diamond sample was grown on a {100} HPHTsynthetic substrate in five different stages. The gas flow.rates were36/600 sccm (standard cubic centimetres per minute) of CH₄/H₂ and thesubstrate temperature was 883° C. Nitrogen and silane were supplied togive the concentrations in the process gases listed in Table 4 below forthe different stages in the growth process. Growth was terminated whenthe total thickness of CVD growth was 1.2 mm.

A {100} polished cross-sectional slice was processed from the sample toenable the properties of the layers to be studied. Distinct layerscorresponding to the different stages of growth could be clearlydistinguished in luminescence images of the slice recorded using aboveband gap excitation. They were also easily identifiable incathodoluminescence images recorded using an SEM equipped with an OxfordInstruments low magnification cathodoluminescence imaging system. AMonoCL spectrometer was used to measure the intensity of 235 nm freeexciton luminescence emitted by each of the layers under electron beamexcitation. Table 4 lists the gas phase silicon and nitrogenconcentrations, the resulting silicon concentrations measured using SIMSand the free exciton luminescence intensities measured relative to astandard sample of high purity CVD diamond. TABLE 4 Si conc. N conc.Free exciton CL Silane Nitrogen (SIMS) (SIMS) intensity relative toLayer (ppm) (ppm) (ppm) (ppm) high purity standard 1 0 0 <0.03 <0.5 1.012 4 0 0.73 <0.5 0.63 3 4 6 1.21 <0.5 0.40 4 4 10 2.45 0.6 0.21 5 4 144.36 0.7 0.23

When the slice was viewed under an optical transmission microscope theCVD growth was uniformly colourless except for the final layer which wasfound to be slightly grey. Absorption spectroscopy at 77 K indicatedthat, in addition to the well known Si-related line at 737 nm, thisfinal layer also showed absorption lines at approximately 945.3 nm,830.1 and 856.8 nm and a broad rise in absorption between these linesand approximately 750 nm. The grey appearance results because thishigher level of absorption is observed across the whole of the visiblespectrum. Absorption coefficient spectra were derived from absorbancemeasurements (after subtraction of the calculated reflection lossspectrum). For all positions within the first four layers of the samplethe absorption coefficient was found to less than 1 cm⁻¹ between 350 and800 nm, only rising above 0.8 cm⁻¹ at 737 nm. For the final layer, theabsorption coefficient was found to lie between 0.9 cm⁻¹ and 2.1 cm⁻¹,only rising above 1.5 cm⁻¹ at 737 nm.

EXAMPLE 11

A layered single crystal CVD diamond sample was grown on a {100} HPHTsynthetic substrate in four different stages. The gas flow rates usedwere 250/60/4000 sccm of CH₄/Ar/H₂ and the substrate temperature was825° C. Table 5 lists the N₂ and B₂H₆ process gas concentrationssupplied for each stage of growth, along with the correspondingconcentrations (as measured by SIMS) of atomic nitrogen and boron ineach layer of the sample. The total thickness of the CVD materialdeposited was 1.0 mm. TABLE 5 mean solid gas phase N₂ gas phase phasenitrogen mean solid phase Layer (ppm) B₂H₆ (ppm) (ppm) boron (ppm) 1 0 0Undetectable <0.1 2 10.4 0.029 0.3 1.3 3 10.4 0.019 0.3 1.0 4 10.4 0.0085.0 1.5

A {100} polished cross-sectional slice was processed from the sample inorder for the layers to be studied. FIG. 22 shows a low magnificationoptical microscopy image of this sample, in which the CVD growth stagesare indicated.

The layer corresponding to the first stage of growth was performed underprocess conditions for high purity CVD diamond growth, and as such thisthin initial layer is of high colour. For the subsequent stages ofgrowth the nitrogen level in the gas phase is set at a level whichmimics an uncontrolled air leak into the gas system, in which the sizeof such a leak would introduce sufficient nitrogen to lead to very poorcrystalline quality CVD material. FIG. 22 shows that reasonable colourand good crystallinity is maintained in the layers corresponding to the2^(nd) and 3^(rd) stages of growth, despite the presence of a highconcentration of nitrogen in the process gas mixture. This is due to thecontrolled amounts of diborane added to the process, which amelioratethe negative effects of the nitrogen. In particular, the boronincorporated into the material inhibits excessive surface roughening andsubsequent material degradation and in addition provides compensation ofthe nitrogen donors. In the 4^(th) stage of growth the diborane in theprocess gas mixture is now below the level at which surface rougheningis inhibited and in which the boron incorporated into the material fullycompensates nitrogen. Thus the material turns black and the crystallinequality is poor. The measured increase of both the boron and nitrogenconcentrations in the material can be explained by the increased surfaceroughness, which leads to a general increase in impurity uptake.

EXAMPLE 12

A layered single crystal CVD diamond sample was grown on a {100} HPHTsynthetic substrate in seven different stages. The gas flow rates usedwere 250/60/4000 sccm of CH₄/Ar/H₂ and the substrate temperature was805° C. Table 6 lists the N₂ and B₂H₆ process gas concentrationssupplied for each stage of growth, along with the correspondingconcentrations (as measured by SIMS) of atomic nitrogen and boron ineach layer of the sample. The total thickness of the CVD materialdeposited was 1.2 mm. TABLE 6 ratio of boron- % free mean mean boundexciton CL gas gas solid solid exciton CL intensity phase phase phasephase intensity to relative to N₂ B₂H₆ nitrogen boron free exciton highpurity Layer (ppm) (ppm) (ppm) (ppm) CL intensity standard 1 0 0 0.110.13 0.06 14.96 2 5.2 0.037 0.18 1.19 1.03 11.00 3 10.4 0.037 0.61 1.511.28 8.42 4 16.6 0.037 0.98 2.28 1.6 8.96 5 22.8 0.037 1.25 2.7 1.918.08 6 1.0 0.037 0.2 2.87 2.26 5.82 7 29.1 0.037 2.6 3.89 2.24 6.54

In this example, the boron incorporation in the material increasedsteadily as a function of growth time, despite a constant concentrationof diborane in the process gas mixture, which may be attributable tosmall changes in reactor conditions, such as temperature. In each layerof growth the boron incorporation in the material is greater than thenitrogen incorporation, such that full compensation of nitrogen donorsis achieved. Thus, similarly to Example 10, the material is able totolerate relatively high amounts of nitrogen without degrading, as longas full compensation of nitrogen donors is maintained.

Excitonic spectra were measured in each layer of the sample, using anSEM system equipped with an Oxford Instruments cathodoluminescence (CL)system. The spectra were recorded at liquid nitrogen temperature, andfrom each spectrum the ratio of the boron-bound exciton intensity to thefree exciton intensity was calculated. In addition, the free excitonintensity was compared to the free exciton intensity in a standardsample of high purity CVD diamond. The boron-bound exciton intensitycorrelates to the increasing boron concentration in the material, asexpected. In addition, the free-exciton intensity relative to the highpurity standard sample decreases as a function of increasing boronincorporation, which is consistent with an increase in the boron boundexciton intensity. It is remarkable that the free-exciton intensityrelative to the standard high purity sample is as high as it is, giventhe levels of nitrogen in the material. For example, for a CVD samplewithout boron, but with only 0.08 ppm of nitrogen, a free excitonintensity <10% relative to the standard high purity sample has beenmeasured. This is further evidence of the ameliorating effect of boronin the presence of high levels of nitrogen: the effect of boron is toprevent the surface roughening associated with high nitrogen uptake,which inhibits the uptake of other point defects which would ordinarilyextinguish the free exciton intensity.

This above-mentioned compensation effect also leads to high colourmaterial, relative to the case in which nitrogen had not been present.In layer 6, of Table 6, in which the gas-phase nitrogen is reduced to 1ppm, the layer is distinctly visible due to its blue colourationcompared to the relatively high colour of layers 5 and 7.

EXAMPLE 13

A 50 mm diameter molybdenum substrate was prepared for the growth of apolycrystalline CVD diamond layer. Prior to commencement of growth, thegrowth environment was determined to have an uncontrolled nitrogenconcentration of 2.5 ppm as measured using gas chromatography. Such aconcentration of nitrogen would normally result in a polycrystallinediamond layer of poor quality. An addition of silane, made as 100 ppmsilane-in-hydrogen, was made such that the concentration of silicon inthe gas phase was approximately 1.5 ppm. Growth was commenced using aplasma-assisted CVD process with a gas composition of H₂/Ar/CH₄ of600/10/23 sccm, an addition of 9.5 sccm of 100 ppm SiH₄ in H₂(equivalent to ˜1.5 ppm silicon in the gas phase), at a temperature of880° C. and a pressure of 200×10² Pa.

Growth was continued for over 75 hours and a polycrystalline layer wasremoved with a thickness of approximately 500 μm when measured using amicrometer with pointed anvils. The appearance of the layer was lightgrey and the crystal quality was judged to be good with no evidence ofporosity.

Several 10×10 mm squares were laser cut from the layer and processed sothat the optical properties could be measured. The material wastransparent at visible wavelengths and had a slight grey colour.

1. A method of producing a CVD single crystal diamond layer comprising:(i) providing a substrate; and (ii) adding into a CVD synthesisatmosphere a gaseous source comprising silicon.
 2. A method of producinga CVD single crystal diamond layer, comprising incorporating siliconinto a synthesized diamond material in a concentration less than 10 ppm.3. The method according to claim 2, wherein the silicon is incorporatedinto the synthesized diamond material in a concentration greater than0.0001 ppm.
 4. The method according to claim 3, wherein theconcentration of silicon in the majority volume of the diamond layer isless than or equal to 2×10¹⁸ atoms/cm³.
 5. The method according to claim3, wherein the addition of silicon reduces an adverse effect on aproperty of the produced diamond layer caused by the presence of animpurity atom type.
 6. The method according to claim 5, wherein theimpurity atom type is nitrogen.
 7. The method according to claim 6,wherein the property is colour and adding silicon produces a CVD diamondlayer having high colour.
 8. The method according to claim 6, whereinthe property is free exciton emission of the diamond layer and addingsilicon produces a CVD diamond layer with an increased normalized freeexciton intensity compared to a method where silicon is not added. 9.The method according to claim 6, wherein the property is at least oneof: carrier mobility; carrier lifetime; and charge collection distance,and adding silicon produces a CVD diamond layer with an increase incarrier mobility, carrier lifetime and/or charge collection distancecompared to a method where silicon is not added.
 10. The methodaccording to claim 3, wherein the majority volume of the diamond layerhas at least one of the following features: a) an absorption spectrummeasured at room temperature such that the colour of a standard 0.5 ctround brilliant would be better than K; b) an absorption coefficient at270 nm measured at room temperature which is less than 1.9 cm⁻¹; c) anabsorption coefficient at 350 nm measured at room temperature which isless than 0.90 cm⁻¹; d) an absorption at 520 nm of less than 0.30 cm⁻¹;or e) an absorption at 700 nm of less than 0.12 cm⁻¹.
 11. The methodaccording to claim 3, wherein the diamond layer is formed into agemstone having three orthogonal dimensions greater than 2 mm, where atleast one axis lies either along the <100> crystal direction or alongthe principle symmetry axis of the gemstone.
 12. A CVD diamond layerproduced by the method of claim
 3. 13. A CVD diamond layer comprisingsilicon, wherein the diamond layer has high colour.
 14. A CVD diamondlayer comprising from 10¹³ to 2×10¹⁸ atoms/cm³ of silicon atoms in themajority volume of the diamond layer.
 15. The CVD diamond layeraccording to claim 12, wherein the layer has a thickness of greater than0.1 mm.
 16. The CVD diamond layer according claim 12, wherein the layerhas a birefringence of less than 1×10⁻³ over a volume greater than 0.1mm³.
 17. An optical element comprising the CVD diamond layer producedaccording to the method of claim
 3. 18. An electrical element comprisingthe CVD diamond layer produced according to the method of claim
 3. 19.An electronic element comprising the CVD diamond layer producedaccording to the method of claim
 3. 20. A gemstone comprising the CVDdiamond layer produced according to the method of claim
 3. 21. Agemstone comprising the CVD diamond layer produced according to themethod of claim
 11. 22. A CVD single crystal diamond layer producedaccording to the method of claim 3, and having a clarity of at least SI1on the GIA gem grading scale.
 23. The CVD diamond layer according toclaim 15, wherein the layer has a thickness of greater than 1 mm.